It was the same as today: rice and vegetables, including vegetables and fruits we do not have here, or in Europe. They also grew poppies because opium was allowed and used back then.
But most of all: mulberry trees, for their worms. Their worms which gave long threads of silk when they were still in their cocoons. And in Xi'an, it was THE agriculture: mulberry trees and not any mulberry trees: the white mulberry tree because it yielded a much better silk that was ready to be tainted. The production of silk was a secret. No one could export it. It represented the revenue by excellence of the country. All the other countries wanted silk from Xi'an and so they built the silk road and many commercial roads so as to be able to go get the silk. At times, according to where the merchants were, they were no roads and they had to walk through dangerous passes which we can sometimes see in Chinese paintings with clouds and rain or snow. It was so dangerous, but then silk was expensive and merchants could make a lot of money out of silk.
The also had bamboos with which they made alot of things.
Their fruit, exotic, were also very popular and grown carefully. It represented a revenue.
They also grew medicinal herbs which also were sold along with the silk and the fruits. Look it up on Google: ancient Chinese agriculture, and see what they say. I just say all that out of my studies about the silk road. Good luck!
