The diagnosis analysis for the development mechanism of a cyclone over ChangjiangHuaihe and rainstorm that occurred from 25 to 27 May 2013 was studied utilizing the NCEP/NCAR reanalysis data. The results indicate that positive vorticity advection in upper level, warm air advection in lower level, the divergence located in the right side of the entry of upper jet and convergence located in the left side of the exit of low jet are the most important physical factors for the developing of the cyclone. The development of cyclone is consistent with the mpv1 and mpv2 of moist potential vorticity. Formation and development mechanisms of the cyclone correspond to development of vorticity column troposphere, and the download of positive vorticity advection in upper level intensifies the vortex in lower troposphere. The downward extension of moist potential vorticity could have increased the positive vorticity in the lower level and induced cyclonic circulation. The rainstorm triggered by the cyclone is located in the left front of moving track of the cyclone. Coupling structure of intensity convergence and positive vorticity center enhances the convergence of water vapor and energy, which creates the conditions for the maintenance of rainstorm.