Typhoon“Longwang”, which landed on Fujian Province on October 2, 200 5, caused a severe heavy rain over Fuzhou. Based on satellite pictures and Dopp ler Radar data, diagnostic analysis is conducted. The result indicates that the southwest flow in the southeastern part of typhoon is forced to split into two b ranches by the impact of Taiwan's landform after it entered into Taiwan Strait. The two branches of air flow join together again over Fuzhou. Evidences suggest that the developing mesoscale vortex triggered by favorable surrounding conditio ns is the main reason producing this heavy rainfall. In addition, asymmetry sadd lebacked structure of θe vertical profile, the upper negative and lower positive moist potential vorticity, as well as the highlevel divergence and lowlev el convergence provide dynamical and thermodynamical conditions for occurrence and development of this heavy rain.