A rare extreme rainstorm occurred in Sichuan Basin on 21 May 2018. Both subjective and objective forecasts failed to capture this disastrous event. In this study, the largescale circulation, the triggering and developing mechanisms of mesoscale systems and the possible causes of forecast biases are analyzed based on the observational and reanalysis data. During this extreme rainstorm event, the westward extension of the western Pacific subtropical high and the abnormally strong Mongolia cold vortex jointly led to the southward invasion of the cold air. The water vapor and unstable energy in the southern Sichuan Basin were extremely stronger than normal, which favored the occurrence of convective heavy precipitation. The special topography near the Sichuan Basin was closely related to the triggering and maintaining of mesoscale convective systems. The northerly wind converged and was lifted up over the unique “bell mouth” terrain, forming a mesoscale convergence line and mesoscale low around the gorge. The temperature gradient on the west and south edges of the basin was increased due to the basin terrain and upwind heavy rainfall. Then, the ascending motion was induced in the large temperaturegradient area. The persistent convergence of the northerly wind and the topographic obstruction made the convective systems move slowly, resulting in the increase of accumulated precipitation. However, the convections in the southern basin were mainly affected by the synopticscale systems. The underestimate of convective precipitation by the EC model was possibly caused by the deviation of wind in the lower troposphere in the basin and the undetailed description of the basin terrain.