In order to improve the accuracy of rainstorm forecast and reduce the disaster losses caused by rainstorm, based on the ground conventional meteorological observation data, TBB (Black Body Temperature) data from satellite images and reanalysis data from National Centers for Environmental Prediction (NCEP), the synoptic causes of a strong convective rainstorm in Yunnan in August 2017 were analyzed. The results show that the eastward movement of 500 hPa trough, the southward movement of 700 hPa shear line and the westward movement of surface cold front are the synoptic background of this precipitation process. The mesoscale convective systems (MCS) on the Meso-α and Meso-β scales directly trigger the convective rainstorm. The heavy rainfall generally happens in the region with a high gradient of TBB. The MCS is closely related to 700 hPa wind shear line where is located to the east of Mid-Yunnan. The MCS is elliptically-shaped, developing along the neighboring and backside of the wind shear line. After the wind shear line getting close and cross over the Ailao Mountain, the MCSs distribute in a belt from northwest to southeast, and develop in front of the wind shear line. The wind shear line moves fast during the daytime before crossing over the Ailao Mountain, mainly producing thunderstorm weather while it moves slowly at nighttime, and the rainfall is strong. The forecast of strong convective rainstorm should focus on the large value area of water vapor flux convergence and the area where the temperature difference between 800 hPa and 500 hPa is greater than 20 ℃. During the heavy rainfall, the whole layer atmosphere is ascending, and the heavy rainfall area maintains the dynamic pumping mechanism of convergence at lower level and divergence at middle and upper levels.