Abstract: The South China monsoon precipitation and the monsoon over South China attract much attention. Using the NCEP reanalysis data and precipitation data of stations and grid, the relationship between the South China monsoon precipitation and atmospheric circulation are analyzed, and then a monsoon index of South China which can reflect the intensity of South China monsoon precipitation in June is established. The monsoon index of South China is defined based on the difference of 850 hPa vorticity between the subtropical Northwest Pacific near the Philippine Islands and the South China area. Then the relationship between this index and the South China monsoon precipitation are explored. The results indicate the monsoon index of South China has very good regional representative, the positive correlation coefficient between the monsoon index of South China and grid daily precipitation of Asia is mainly located in South China. The monsoon index of South China could fairly describe the inter-annual variability and extreme year of the precipitation. When the monsoon index of South China is stronger (weaker), the precipitation of South China is more (less) correspondingly, and extreme of the monsoon index of South China also means the extreme of precipitation. The difference of the precipitation over South China between high and low monsoon index of South China is significant at 0.01 level. On inter-decadal scales, the decade in which the monsoon index of South China is stronger (weaker) is also the decade in which the precipitation of South China is more (less).The relationship between the monsoon index of South China and the atmospheric circulation are also studied. The results indicate that the monsoon index of South China also includes the information of the important influencing systems such as southwest monsoon, the Pacific Subtropical High, the westerly trough over the middle-high latitudes, and so on. In the southeastern part of South China, there is abnormal anticyclonic circulation in lower and middle atmosphere, its center locates in the oceans of the east Philippines and its western end of the ridge locates in the Indo-China Peninsula. In the lower atmosphere at 850 hPa, there are abnormal depression circulations in South China. However, there is an abnormal westerly trough in the southwest China in the middle-higher atmosphere at 500 hPa. The monsoon index of South China is defined by the characteristics of circulations affecting June precipitation in South China. The changes of June precipitation in South China can better be described by the monsoon index of South China, and the characteristics of the monsoon circulation system affecting the precipitation in South China can also be expressed, demonstrating a concise objective and quantitative indicator for studying monthly scale monsoon and precipitation in South China. Therefore, the monsoon index of South China has significance of operational use.