A heavy rainfall occurred at Chengdu Shuangliu Airport on 21 July 2017 was studied based on dataset from ground-based Doppler radar, automatic weather stations and microwave radiometer, et al. The results show that the ambient conditions and dynamic forcing prior to the event were weak and characterized by small value of Convective Inhibition(CIN), moderate Convective Available Potential Energy(CAPE),low Lifting Condensation Level(LCL),deep warm layer and moist southerly flow. The initial convection storms were triggered mainly by a convergence line formed by the warm flow from the cities and cold down-lope wind in front of mountains, which combined with the intense temperature gradient. And then the outflow of the cold pools located southerly impacting with the warm,moist boundary layer introduced the other convective storms. The micro-physical analysis indicates that the heavy rainfall was mainly composed of high-precipitation-efficiency warm rain process supported with low-centroid cumulonimbus structures. It also suggested that the forward-propagation storms have played a big role in the local heavy rainfall with intensifying southerly flow and dramatic water vapor supply.