Abstract: In this paper,the rain and clear sky observations in Beijing by the Royal Observatory of the Ching Dynasty were used and combined with instrumental records to reconstruct rainy season from 1724 to 1978.Its characteristics and variations were discussed and concluded as follows:1.There were 6 patterns of rainy season in Beijing during the last 255 years,and each of them prevailed in a certain period of time.2.The begining date of rainy season in Beijing in the average was in the 3rd.decade of June,ranging from the early part of May and 3rd.decade if August.The ending date of it in the average was in 2nd.decade of August ranging from the last part of June to 3rd.decade of Sept.The average length of rainy season was 50 days,ranging from 10 to 115 bays.The number of rainy day in the rainy season was 32(average),ranging from 9 to 82 days.