Abstract: By using the NCEP/NCAR daily reanalysis data, NOAA NCEP Climate Prediction Center Merged Analysis of Precipitation (CMAP) pentad-mean precipitation data and daily precipitation data of 740 stations in China, the authors analyze the similarities and differences of the short-term position variation of the Western Pacific Subtropical High (WPSH) and its probable formation cause during large-scale continuous torrential rain in South China pre-flood period and the Changjiang－Huaihe River valley Meiyu period. Results show that during the torrential rain in South China and the Changjiang－Huaihe River valley, in comparison with the same period climatic mean, the WPSH is exceptionally strong and its position is abnormally south-westward. During the torrential rain in South China, the apparent heating source and apparent moisture sink are exceptionally strong over South China on the northwest side of WPSH and the Bay of Bengal to the west of WPSH. During the torrential rain in the Changjiang－Huaihe River valley, the apparent heating source and apparent moisture sink are also exceptionally strong over the Changjiang－Huaihe River valley on the north side of WPSH and the Bay of Bengal to the west of WPSH. Using the complete form of vertical vorticity tendency equation, the authors study the impact of diabatic heating on the short-term position variation of the WPSH. Conclusions indicate that the short-term position variations of WPSH are affiliated with the diabatic heating and those configurations. The heating field over South China which is on the northwest side of WPSH may force an east-southward withdrawal of WPSH during the torrential rain in South China. And during the torrential rain in the Changjiang－Huaihe River valley, the heating field over the Changjiang－Huaihe River valley on the north side of WPSH is unfavorable to the WPSH moving northward, and the heat source over the Bay of Bengal to far west of WPSH may induce a westward extension of WPSH. The coaction of the two results in the maintenance on the south side of the Changjiang－Huaihe River valley and westward extension of WPSH.