Based on the NCEP/NCAR global reanalysis daily data and precipitation data from Chinese meteorological stations during 1979 to 2013, a new Asian Tropical Summer Monsoon Index(TSMI) was defined by combining the dynamic factor (southwest wind) and the thermodynamic factor (OLR). Results show that the index can well describe the inter-annual variation of the summer monsoon, as well as the abnormal change of summer precipitation in southern China and the characteristics of eastern Asian atmospheric circulation. In stronger (weaker) monsoon years, the Indian summer monsoon trough and the South China Sea summer monsoon trough deepen (weaken), so the southwesterly (easterly) wind anomaly control the bay of Bengal area, meanwhile, cross-equatorial flow moves north strongly(weakly), the South China Sea monsoon trough closes to the west Pacific (Indo-China Peninsula), and the westerly anomaly is in the south part of the South China Sea area, while the easterly anomaly is in the north part. Another characteristics of the circulation abnormal are the separated (overlapping) pattern between the ridge of West Pacific subtropical high and the South Asia high. These abnormal makes low-level wind diverge (converge) in Yangtze Valley, converge (diverge) in southern China. So there exist clear downdraft motion (ascending motion) over the middle and lower reaches of the Yangtze River, which leads to less (more) in middle and lower reaches of the Yangtze River and is more (less)precipitationin southern China.