how to reach northwestern polytechnical university Chang'an Campus from Xian railway station?

There is no direct bus, you need to transfer.
In the railway station, you need to take the bus No. 40 or 707 to Bianjiacun station. After get off, walk for 250 yards to transfer the bus No. 916 to the university directly.
If taking a taxi, it may cost you CNY90 around.