Abstract:By using basic data, nephanalysis and T213 numerical prediction products, transi tion precipitation was diagnosed and analyzed during key period of defend agains t puff the “5·25” big forest fire in 2006. The study shows that the air tempe rature in fire region of Hulun Buir City is about 3～4 degrees higher than the no rmal. The precipitation is 90%～100% less than the surrounding area. The vapor re source during defend against puff fire is mainly from the Bay of Bengal and low latitude areas of China. The vapor flux value at 700hpa is 19.7g·s-1·cm-1·hPa-1. At the same time, moisture was added in low level of Jianghuai Valley. The transport of positive vortex advection maintains and strengthens the east wind she ar in fire region. There is an increased vertical velocity zone in this area. Th e value is -15.6×10-3hPa·ｓ-1.The vertical distribution of diverge nce field has good suction effect. The guide role of divergence at high level for precipita tion is obvious. Thermodynamic factors increase the precipitation in fire area . Results indicate that abnormal atmospheric circulation results in high tempera ture and drought at the earlier stage of fire. The long distance transport of va por is the most important factor for the transition precipitation. The dynamic r ole is notable during key period of defend against puff the “5·25” big fo rest fire.