A 72hour numerical simulation of Typhoon Damrey is performed by using WRF (V311). The possible mechanism resulted in the rapid intensification is explored. The results show that: (1) Interaction between easterly waves and typhoon’s circulation at upperlevels during the slowly intensify stage induced a broad range of northeastsouthwest direction outflow channel, which increased divergence at upperlevels and was favorable to the intensification; the vertical wind shear reduced in the first 12 hours, and the rapid intensification lagged behind the vertical wind shear decrease.（2）The water vapor flux toward the typhoon’s center at lowerlevels increased magnificently during the rapid intensify stage; the heat flux, latent heat flux and moisture flux, at the sea surface were growing constantly, and the contribution of latent heat flux was greater than that of heat flux. (3) The mechanism of conditional symmtrical instability excited the tilted updraft; the development of slantwise vorticity induced the significant increment of relative vorticity near the typhoon center and promoted the strengthening process.