Abstract: In this paper, a diagnostic study of a spring cyclone developed rapidly was made. The eddy kinetic energy (EKE) budget, eddy available potential energy (EAE) budget and vorticity budget have been calculated by using the OuasiLagrangian scheme during the period 27 to 29 April 1983.The principal results are the following: (1) Advantageous large-scale environments were a very crucial factor for the explosively deepening cyclone. In the early developing stage of the cyclone, the baroclinic process was dominant. And the EKE generation by barotropic process also increased rapidly and the cyclone could be considered as a "quasi-closed" system in its explosive stage: (2) The EAE increased rapidly before the cyclone developed explosively. The EAE generation by the release of latent heat was very small. And,the transformation from average available potential energy to EAE was a main source. (3) The vorticity variations in middle-upper layer of troposphere was most notable when the cyclone developed explosively, In vorticity budget, the effects of divergence term in middle-low layer of troposphere and the large-scale and subgrid-scale vertical motion and vorticity advection in middle-upper layer of troposphere played an important role for the increasing of positive vorticity in the explosive stage of the cyclone.