Based on Potential Vorticity (PV) budget diagnosis and particle track and so on, the causes of intensity evolution of super strong typhoon “Saomai” which impacted China most severely in 2006 is analyzed and studied. The results show that the variations in the vertical wind shear, vorticity and divergence around the center of the typhoon significantly effected the variation in its intensity. The PV dignosis shows the functions of latent heat of condensation, vertical advection and horizonal advection were different while the typhoon intensified and abated. Condensation latent heat plays a dominant role in the lower troposphere at early rapid development stage and before development, so does horizontal advection in the whole troposphere at the rapidly developing stage, and verticl advection in the bottom layer of typhoon at rapidly weakening stage which reduces the potential vorticity, respectively. Lagrangian backward trajectories revealed that the instrusions of air from the southeast at upper level and from the south at lower level contributed a lot to the development of Saomai while the typhoon′s intensity was changing.