The evolution of cloud systems of severe tropical cyclone Koni and the effects on its path and precipitation in central part of South China Sea are studied in detail. The results show that the attraction of convective cloud cluster was responsible for turning southward after Koni's landing. The distribution of vertical motion has close relationship with the path changing of the tropical cyclone in a short period. The tropical cyclone tends to move towards ascending motion. The meso-scale heavy rain occurs in spiral cloud band of a small vortex in the severe convective cloud cluster or in the overlapping area of dense gradient of TBB and surface wind direction convergence.