In terms of the NCEP/NCAR 1970～2000 daily reanalysis data in June, 12 westward extension processes of western Pacific Subtropical High are selected based on the variation of 500 hPa vorticity in the key area （20~30 °N,115~125 °E）. Through composite analysis, the features of the atmospheric circulation evolution from upper to low level of troposphere during the processes of westward extension of western Pacific Subtropical High are investigated in this paper. Results indicate that the increase of the negative vorticity in key area agrees with the westward extension of western Pacific Subtropical High. Another interesting phenomenon is found that the center of South Asia high in the upper level of troposphere is stabile and nearly motionless during the processes of the westward extension of western Pacific Subtropical High, but its eastern ridge strengthens and extends eastward remarkably because the anticyclone moves westward over the southeast Japan, which may lead to the westward extension of Subtropical High in the middle level of troposphere. In the low level of troposphere, the north Australia cold air activity plays an important role in the westward extension of Subtropical High. The flow propagates northward and marches across the equatior, then passes through the trough of summer monsoon over South China Sea and finally gets to the Yangtze River Valley, which sequentially affect the southwest airflow of the west side of Subtropical High.