Abstract: The vertical motions and secondary circulation of an explosively deepening oceanic cyclone, which occured over the Northwest Pacific Ocean and was in conjunction with 200 hPa-level jet stream and has central pressure falls of 33.9hPa/24h, have been computed from seven-level nonliner balaace model and Sawyer-Eliassen-Shapiro Equation for the transverse ageostraphic circulation. The vertical motions are partitioned into contribution from large scale latent heat release, effects of cumulus heating, thermal advection, differential vorticity advection and so on, while the secondary circulation stream function is partitioned into contributions from geostraphic deformation, transfer of momentum and heat in the area of cumulus and diabalic heating. The principal results are the following:large scale latent heat release is very crucial to explosive development of cyclone. If there is enough transfer of moisture, the positive feed-back process between ascent of air and large scale heating would work. The cumulus heating and the transfer of momentum and heat in the area of cumulus played an important role during the explosively deepcning stage. Thermal advection is the initial triggering condition for large scale heating and the conditional instability for the convection of cumulus.