You don't have to buy a ticket in advance. Just go to the window to buy tickets. Unless you encounter a super-popular movie like "33 Days of Lost Love" released on a major festival, you can buy tickets directly in recent days. Before you go to the cinema, look at the opening time of the movie in the cinema before you go, so as not to miss it and go too early.
In addition to the People's Theater, you can also watch movies at the Dida Cinema (the first floor of the church) at the west gate of Zhongshan Park, and the student ID card is also discounted.