Abstract: Based on multiple observations and ERA5 reanalysis data, the causes of persistence and forecast deviation of a heavy rainfall process affecting Yunnan Province from 15 to 19 August 2020 are explored. The results show that the continuous eastward movement of Tibetan Plateau High plays a critical role in the long duration of the event. The dominant synoptic system for the precipitation on 500hPa is the convergence area between the Yunnan-Burma High and the West Pacific Subtropical High, which evolved into the south vortex of the northern trough and eventually developed into the two-high convergence system formed by the Tibetan Plateau High and the Western Pacific Subtropical High. In this process, the high-level cold advection at middle and high latitudes promotes the development of the shortwave trough. The cold air invading Yunnan at the middle level strengthens the stratification instability over it, and the cold air at the low level enhances the uplift of warm and humid air. Under favorable weather conditions, Ailao Mountain in southwestern Yunnan has a significant increase in precipitation in the region. Especially on the windward slope, the positive correlation between altitude and precipitation is good, but the effect of terrain on the increase of precipitation does not always increase with the increase of altitude. The ECMWF numerical model did not predict the westward shift of the two-high convergence system affecting precipitation in Yunnan, resulting in a significantly smaller precipitation forecast from central to southwestern Yunnan.