Based on the NCEP 0.25 ° × 0.25 ° reanalysis data, observation data of Guangxi automatic weather station, FY-2G satellite cloud image and weather radar data of Fangchenggang, the characteristics and causes of extreme wind and rain caused by Typhoon “Ma-on”(No. 2209 ) in Guangxi are analyzed by synoptic diagnosis method. The results show that: 1) This typhoon process had the characteristics of strong wind and rain extremes, obvious locality and short severe rainfall duration. Extreme rainfall occurred after the typhoon landed in Vietnam, while extreme winds appeared before the typhoon landed. The extreme values of wind and rain both were caught in Fangchenggang City. 2) The ultra-low-level southerly and southeasterly jets converged in the area of Fangchenggang. The water vapor and dynamic conditions over the coast and land of Fangchenggang were stronger than those in the offshore sea in the south of Dongxing. At the same time, there were friction convergence and mesoscale convergence lines near the coastline, which are the possible causes for the development and strengthening of the initial strong echo near the coast of Fangchenggang. The southerly warm moist jet was forced to uplift at the coastline and windward slope forming small-and meso-scale convections, which had a significant strengthening effect on extreme rainstorm. The small-and meso-scale convections repeatedly developed in the coast and the piedmont area and moved to the center of the rainstorm, forming the “train effect” and causing high efficiency of rainfall. This was the direct cause for the localized extreme severe rainfall in Fangchenggang. 3) The low-level airflow at Wanweidao Station turned from northerly airflow to southeasterly warm-humid airflow when Wanweidao Station was near the thunderstorm with very strong updraft. The low-level warm-humid inflow was accelerated by the suction of the updraft area when it was about to enter the updraft area, which led to the short-time extreme strong winds on the ground. In conclusion, the offshore intensification and “train effect”"" are the precipitation echo characteristics of extreme severe rainfall in coastal areas, which should be paid attention to in the forecasting and early warning of extreme severe precipitation.