Abstract: Based on conventional meteorological observation, automatic stations, regional rainfall stations, satellite, radar and reanalysis data, a heavy rain in warm sector before cold front occurred in August 2015 were analyzed. The results showed that: This heavy rainfall process happened in the background of westerly trough eastward and the subtropical high southeast retreat, and the strong convective precipitation led by thermal condition in warm sector before cold front was the main contribution of the heavy rainfall. The rainstorm appeared in the overlapping area of the significant divergence zone at the right rear of the upper-level jet, warm shear line at 700hPa, cold shear line at 850hPa, surface dry line and the region where Showalter Index was less than -2℃. The mesoscale cyclone vortex on 850hPa shear line together with the shear lines of extreme wind of automatic stations and mesoscale vortex directly triggered the rainstorm. Due to the deviation error of the model forecast in position of the 850hPa shear line, the mid-lower flow configuration was misled from a forward-tilting to be a backward-tilting structure, which directly affected the model accuracy in thermal condition forecast, and the result was that enhanced the Western dynamic condition and weakened the Eastern thermal condition, which ended up with a deviation error of heavy rainfall area forecast. The key of heavy rainfall in warm sector before cold front forecast was the conditions of triggering the release of unstable energy. For such rainstorm forecast, we should pay more attention to the wind field changes in boundary layer and strengthen the comprehensive analysis and application of the monitoring data.