Using the MICAPS routine data, mesoscale automatic station data and the products of ECMWF and NCEP numerical forecasts, by means of the weather diagnosis analysis method, the causes of the super typhoon “Rammasun” intensification in the near sea and its leading severe weather disaster in Guangxi are analyzed. The results show: (1) the “Rammasun” intensification in the near sea is related to the following factors: the anomaly higher sea temperature in the area where it passed, the strengthened southwest monsoon jet, and the small vertical wind shear; (2) Rammasun met the monsoon jet near the offshore of Wenchang in Hainan, which made the latent heat enlarged near the typhoon and the warm heart structure of the typhoon became completed, which is an important reason for Rammasun intensification; (3) the abundant water vapor transport and convergence and strong ascending motion were the important reasons for the abnormal rainstorm.