Obviously, China could be thought to have racial divisions within its own culture from the very beginning, for example Xunzi lists off a few near the beginning of his essay "On Learning." However, it seems he would believe these races to all be equal, as he notes that they all have the same nature at birth, while learning makes them distinct. However, fast forward to the China near the end of the 19th century, and you have people like Kang Youwei who believed that certain races were of lesser intelligence. Hence, we can see a huge difference in understanding of race from Xunzi to Kang Youwei.
However, I have been taught (informally) that race is sort of a western invention, or at least distinctions based on race (i.e a racial hierarchy). Would it be fair to say that Kang Youwei would have adopted his views on race from the West, or could it be said that China has its own distinct evolution of view on race?
This doesn't answer your historical question, but it might be worth clearing up what people mean today by saying that race is a social construct.
There are different ethnicities in the world, but our decision to group different ethnicities together into large umbrellas of "race" is the social construct that most people are talking about.
We put Ashkenazi Jews, Greeks, Germans, Basques, and Slavs in the same "white" category. Are they so similar to each other, and so different from Arabs, Persians, and Kurds, that the categorization makes sense? Or does it all descend from somewhat arbitrary line drawing?
Does the U.S. Census Bureau's categorization of South Asians, East Asians, and Pacific Islanders as a single category of "Asian/Pacific Islander" follow some kind of principled system, or is it an arbitrary categorization that makes sense only in the context of American society?
It's also worth noting that we tend to account for shared culture and language when drawing the lines of ethnicity in the first place. So meaningfully separating culture from race is difficult to begin with, and I'm not sure what you'd be able to do with the isolated variables.