When we think of the soviet union we often think of poor farmers with barely enough to eat but was this really the case? if so what kept them from moving foward and having a pretty successful economy such as china today?
I'm going to take this question as "Why did Gorbachev's attempts at economic liberalisation fail?" I'll try to answer with reference to China, although I'm less familiar with Deng Xiaoping's policies.
It comes down to a number of factors:
China still had a very strong peasantry when economic liberalisation began in the late 70s, and major aspect of encouraging growth in China was to de-collectivise land (letting the peasants own their land, at least to some extent), grow what they wanted and sell it.
By contrast, the peasantry in the Soviet Union had been weak since Stalin's collectivisation in the 1930s. Russian agricultural workers were largely uninterested in de-collectivisation because they already owned small private plots of land, and they already had small local markets at which to sell their goods. Indeed, the collective farms were something of a boon, as most farmers stole supplies or produce from the collective farms for themselves, while the Soviet government footed the bill. What you describe in your question as, "poor farmers with barely enough to eat" was not really the norm by the 1980s. This is not to say that the Soviet farmers had a particularly great quality of life compared to their Western counterparts, but it was possible to live fairly comfortably.
Gorbachev's Peretroika attempted to reduce Party control over the economy, including allowing the liberalisation of salaries (businesses could decide what to pay their employees), and reducing the payments of businesses to the State. However, this policy did not work as intended. While businesses could retain more of their profits, there was a lack of consumer goods in the Soviet Union (at least on the open market), and therefore little to spend money on. The net result was a siphoning-off and hoarding of profits in many factories, so the money that the State was no longer receiving didn't find its way back into the economy.
At the same time, the Soviet government invested in a lot of new equipment for industry, but implementing new technology takes time and the effects of this investment were not felt soon enough (ironically, it would be the new capitalists of the 90s that would benefit from all this Soviet investment).
As the Gorbachev attempted to liberalise its economy, the government found itself losing funds from two major sources.
First of all, the Soviet Union's greatest export, oil, suffered a catastrophic drop in price, falling to below $10 a barrel (that's 1986 US dollars) in 1986.
At the same time, Gorbachev instituted a campaign against alcohol, which had become a major social problem during the Brezhnev era. However, alcohol sales were also a significant source of the income for the Soviet government. The anti-alcohol campaign not only reducing another source of revenue, but had a drasticly negative effect of Gorbachev's popularity.
China in the late 70s had no real welfare system to speak of, whereas in the Soviet Union there were subsidies across the board: education, transport, and housing were all significantly subsidised, while the government was also paying out a lot in pensions. So, while the income of the government was reducing by falling oil prices, alcohol sales and the reduction in payments from businesses to the State, it still had significant outgoings.
The Soviet Union's international position was far less favourable to economic liberalisation than China. The USSR had yet another major cost in the form of its military, and the Soviet military-industrial complex had grown significantly under Brezhnev as he had pursued nuclear parity with the United States. The Soviet Union was also still involved in costly military action in Afghanistan.
China was also plainly more attractive for Western investment than the USSR: not only were they growing closer to the United States, but China could offer far cheaper labour than the USSR (whose citizens were accustomed to a higher quality of life than the Chinese).
Whilst Deng Xiaoping liberalised China economically, he did not liberalise socially or politically (or, not nearly to the same extent). Freedom of speech was restricted and the personality cult of Mao Zedong was maintained.
In the Soviet Union, Gorbachev pursued a policy of Glasnost (openness), allowing much more free speech (especially in media) and also allowing for the first time dissenting political movements. Most significantly, nationalist movements were permitted to exist openly. Gorbachev became unpopular for his anti-alcohol campaign, and also for the failure of many of the economic policies described above. Ultimately, the unpopularity of the government combined with the newly-legal dissident movements lead to the dissolution of the Soviet Union.
Some further reading:
Yurchak, Alexei, Everything was Forever, Until it was No More: the Last Soviet Generation
Zubok, Vladislav, A Failed Empire: the Soviet Union and the Cold War, from Stalin to Gorbachev