A squall weather process over the northern Zhejiang Province is analyzed. The results show that the squall took place under the condition of upper forward trough and in the instability area near the sub tropical high. The warm and moist temperature advection and the continious rising surface temperature over northern Zhejiang province stored a great energy of instability, and the upper cold temperature advection greatly promoted the instability. The water vapour analysis revealed that the warm and moist southwest air current at 850hPa transported water vapour to the squall occarence place where is just the interface of dry and moist air or the front zone of dew point, but the water vapour at 500hPa and 700hPa was transported by northwest air current. This is the squall weather process's feature.