i want to see the great wall of china, Terrecotta Army. When is the low season?

In CN, the best place to see the Great Wall is Beijing. There are many preserved sections, such as Badaling, Mutianyu and Juyongguan. You are advised to visit the both cities during Sepember to October. The weather is quite moderate for a trip and the scenery during that time can be very joyful. Notably, you should try to avoid the public holiday from Oct. 1st~7th and weekends. Other times, there won't be too many visitors.
Hope you enjoy your time there!