Based on 1°×1° NCEP 6 hour reanalysis data, satellite cloud data and Doppler Radar Data, used synoptic diagnosis method,the reasons of a heavy rainstorm which caused by typhoon "WIPHA" over southchina sea in Fangchenggang were diagnosed.The conclusions are as follows:①The weak steering flow of typhoon , the slow typhoon center moving speed,the intensity of typhoon weakened slowly and typhoon center made landfall in Fangchenggang what was the primary cause of a heavy rainstorm;②Typhoon center clouds stranded over Fangchenggang a long time,what was the direct cause of a heavy rain;③Typhoon precipitation was convective precipitation and the precipitation efficiency was high,what caused large area heavy rain in Fangchenggang;④Fangchenggang was always located at the 925 hPa water vapor convergence center and 850 hPa vertical upward movement center or near, which was conducived to the accumulation and uplift of a large amount of water vapor to form convective cloud system and produce persistent heavy precipitation;⑤The atmospheric convection was very unstable during the period of "WIPHA" in Fangchenggang and heavy rainfall mainly occured near the negative center of wet positive pressure (MPV1) ,which had guiding significance for the heavy rainfall area;⑥There was a wide southeast wind jet belt behind the "Weipa" Circulation over Fangchenggang, which was forced to rise in front of Shiwan mountains to form convection and had an obvious effect on the occurrence of heavy rain in Fangchenggang.