Abstract: Based on ECMWF monthly reanalysis information, monthly rainfall and air temperature data from 160 primary stations in China from 1958 to 2002, it were discussed that the characteristics of convection and zonal wind over plateau during summer and the inter-annual variation and inter-decadal variation of plateau summer monsoon.Further, we define a new plateau summer monsoon index and analyze the relationship between the plateau summer monsoon index (PMI) and summer climate in China.The main conclusions are:(1) The plateau summer monsoon index (PMI) was defined using the difference of anomalous wind between the (27.5~30°N, 80~100°E) and (35~37.5°N, 80~100°E) at 600hPa by the averaged from Jun to Aug.The phenomena have been found that the opposite variation occurred between the latitude (27.5~30°N) and (35~37.5°N) over plateau for the anomalous zonal wind at 600hPa.The PMI corresponds with summer climate over China in distinctive patterns. (2) The plateau summer monsoon had an increase trend from 1958 to 2002.Before the middle 1960s the plateau summer monsoon is in its powerful stage, afterwards it weakend until the end of 1980's and became strong again.The remarkable inter-annual variation and inter-decadal variation of the plateau summer monsoon occur throughout 45 years. (3) There existed obviously positive correlation between the plateau summer monsoon and the precipitation in the west of Xinjiang, the east and south of plateau, the northwest of Yunnan, the east of Sichuan and Chongqing Basin, and the downstream of Yangtze River in summer, and obviously negative correlation between the plateau summer monsoon and the precipitation in the west of Sichuan and Chongqing Basin, Gansu, Hetao, North China, and South China.Similarly, there existed obviously positive correlation between the plateau summer monsoon and the air temperature in the north of Xinjiang, the north of Qinghai, the northeast of plateau, Hetao, Gansu, and South China in summer, and obviously negative correlation in the west of Xinjiang, the east of Sichuan and Chongqing Basin, and the downstream of Yangtze River.In summary, it indicates that the plateau summer monsoon has an important impact on the intermediate deck aerosphere circulation climate. (4) Comparison of the overall performance of the new plateau summer monsoon index and the one defined by Tang (1984) shows that the plateau summer monsoon index has a better instruction significance to the summer precipitation and the temperature of Sichuan and Chongqing Basin.