Can we travel in one day by bus from Shanghai to XiAn?

Yes, you can. Go to SH General Bus Station to take the direct long-distance bus to Xian. There are only two in operation every day, leaving at 09:00 and 11:00. It takes 16-17h and the bus fare is CNY380 per person.

Upon arrival at Xian, take a taxi to local Railway Station, at the east square of which you can take bus no. 914, 915 or Tourism line 5 (no. 306) to the Terracotta Museum directly within around 75min.