Using the PSU/NCAR MM5,control and dry experiment for the explosive cyclone process of 19th to 21st December in 1981 over the Western Pacific are performed.The results show that the tropopause and isothermal surfaces were raised by latent heat release in the upper level,and the disturbance trough was formed at the upper level:due to the adiabetic cooling of the ascending flow.Its adaption process resulted in the inertial instability in the upper level.Inertial instability always existed in company with the strong inertial stability area,furthermore,the acceleration field caused by these two factors gave rise to air convergence and divergence in the upper level,which made the cyclone strengthened and precipitation enhanced.The reinforced precipitation in turn caused latent heat release enlarged again,leading to inertial instability augmented.The process of positive feedback was favorable for the deepening of the cyclone.