Chang 'an Campus of Northwest University,No. Xuefu Avenue 1, Guodu Town, Chang 'an District, Xi City, with zip code of 7 10 127.

However, there is absolutely no need to write that long. Just write the Xi Chang 'an Campus of Northwest University directly. I even received a letter from Hunan without the word "Xi 'an", O(∩_∩)O haha ~