Dunhuang Cave 390 was constructed in the early 7th century, in which there are 53 preaching scenes painted on the upper part of the four walls and 60 preaching scenes on the middle and lower parts. This paper infers that the upper part is a description of the Fifty-Three Buddhas in Buddhism, while those on the middle and lower parts are a combination of the Thirty-Five Buddhas and the Twenty-Five Buddhas. In the Three-Stage School, practitioners worship six times a day by reciting the names of the Fifty-Three Buddhas, Thirty-Five Buddhas, Twenty-Five Buddhas, and Seven Buddhas. It can thus be inferred that worship is the theme of cave 390, which is likely related to the popularity of the Three-Stage School at that time.