How far from Bejing is xian. ? Is it possible to do a day trip to see terracotta warriors.?

It may be a little bit hard.
To Xian, the distance is around 1400km by taking trains. The airline distance is about 800km.
If you really want to make a trip, you can take a train in the afternoon to reach Xian in the morning and make a day trip and then take a train at night going back to BJ. In this way, your trip can be finished, but the time is a little bit rush.
You may refer to https://www.travelchinaguide.com/china-trains/ to find a suitable train for you.