Abstract: In this paper,it is shown that the differential reflectivity ZDR(dB) is relative to drop size distribution and the spectra shape variation of different truncate diameters. The effects of shape variation of raindrop size distribution on rainfall measurement can be deduced with(ZDR, ZH) dual parameters technique,which is effective to improve the accuracy of rainfall measurement and of the priority of identifying large particles of hydrometeor(distinction hail) from rain region.