The Qing emperors came to Chengde to escape Beijing's summer heat — which is still a good reason to visit. July and August are warm rather than sweltering, with the Mountain Resort's parks at their greenest and the lakes open for boating. Autumn (September–October) is excellent: cooler temperatures, clear skies, and the hills turning red and gold. Spring is pleasant but can be dusty.
Winter is cold and quiet. The mountains around Chengde hold snow from December through February, and visitor numbers drop sharply. Some outer temples reduce their hours. If you don't mind the cold, the resort grounds can be atmospheric and almost deserted.
Avoid the Chinese national holidays (Golden Week, early October) when the site crowds considerably.