By statistical analysis of Doppler radar velocity field data of rainfall processes from June 2003 to October 2005 at Dalian district, the indications of several familiar echo velocity characters of the cloud systems developing into rainfall were obtained. Several criterions were concluded for analyzing the precipitation potential region and the opportunity for precipitation enhancement operation while radar echo showed “s” or bow shape. Combined with EVAD technique and variation to calculate average divergence and vertical velocity, the practical validation showed good results in a precipitation enhancement operation at Dalian district in summer 2006, in that case, the PPI velocity field showed “s” shape and matched well to the average divergence and vertical velocity. 