MICAPS data was used to research the reasons of the path’s deflecting southwe stwards of 0604 severe tropical storm by synoptic and diagnosis analysis, and by the analysis on the storm rainfall caused by the severe tropical storm in Yunna n Province. The physical mechanism of the maintenance of the tropical storm’s s trength after landfall is also discussed. The result indicates that the main reason of the path’s deflecting southwestwards of Bilis w as the leading of northeast wind at the south of high which was formed by the me rging of continental high and the Huanghai subtropical high after its moving sou thwards and strengthening eastwards. Heavy precipitation caused by Bilis happene d at the center of water vapor convergence area which located in the northeast j et region on the front of inverted trough of tropical depression, and the locati on of the precipitation is corresponding better to the wet Q vector convergence area. During the period when the influence of tropical depression existed, the a tmosphere had wet neutral vertical moving structure and the situation was benefi cial for the growth of storm rainfall system. The interaction between Bilis and severe southwest jet for a long time after the landfall of Bilis was the main re ason for its maintenance. With the fading of southwest jet, the precipitation fi nished completely. The southwest monsoon cloud’s getting into the tropical depr ession strengthened the input of water vapor and energy, which was another impor tant reason that maintained the tropical depression and caused the heavy precipi tation. The South Asia high strengthened during active period of the tropical de pression, and the strengthening of divergence field on the upper level, the exis tence of lower level convergence and upper level divergence, and the maintenance of vertical positive vorticity which was related with southwest jet were benefi cial for the maintaining of the tropical depression and the movement tendency of the tropical depression to upper severe divergence area after its landfall.