Abstract: In this paper, a northernward neutercane (8509) in its developing process was analysed by using diagnostic methods. The result was compared with another typhoon (8406) whose early path and intensity were similar to 8509 but not developed when it entered the westerly belt. The results show: (1) Advantageous large - scale environments was a very crucial factor and prerequisite condition for intensification of this neutercane when it interacted with westerlies frontal system; (2) In the early weakening stage of this typhoon, the exchange of EKE (eddy kinetic energy) with the environment was small and barotropic process was dominant. But in its developing stage, the EAPE (eddy available potential energy) increased rapidly. Large amount of EAPE was released and EKE generation by baroclinic process played a significant role. (3) The analyses of frontogenesis in low layer, environmental conditions in upper layer and Q - vector characteristics of mid - latitude area would provide forecasters with a lot of useful information for determining the variations of this neutercane.