The daybyday ridge line positions of the West Pacific subtropical high in the area of 110°to 130°E and 20°to 50 °N are analyzed by means of the NCEP/NCAR reanalysis 500 hPa data from 1958 to 2002. The results show that there is a large positive correlation area in North China, where the precipitation in midsummer (July and August) is related to the number of days with the West Pacific subtropical high intensity being over 5880 gpm and the ridge line being north of 30°N (the correlation coefficient at the center being 0.561 and the significant level being 0.001). In addition, the date of the first day of West Pacific subtropical high crossing 30°N, i.e., the first day of the subtropical high ridge line being north f 30°N in three successive days, is closely correlated with the midsummer precipitation over North China, with a correlation coefficient of －0.385. The midsummer precipitation in North China was heavier when the West Pacific subtropical high moved northward earlier, and vise versa. The results indicate that the number of the days of the subtropical high ridge line being north of 30°N and the date of the ridge line crossing 30°N for the first time in the area of 110° to 130°E can well describe the relationship between West Pacific subtropical high and midsummer precipitation in North China, even in the eastern China, which is important for heavy rainfall forecasting.