The analysis using Gama type raindrop distribution show that the effect of downdrafts W on the relation between the radar reflectivity factor Z and the precipitation rate I should not be neglected when its dimension reaches or exceeds 5×10¹cm·s^-1.In the form Z=AI^b,the coefficient A decreases and the exponent b increases as W increases,which is in agreement with the results from two cases computations well.The influence of the ambient pressure field p,which actually reflects the role of air density,is also analysed and the results indicates that A decreases and the exponent b increases as p decreases.In addition,some results of the discussion on the micro-physics characteristics of the raindrop spectra observed in two cases are presented.