A mixed precipitation weather process by using Parsivel laser raindrops spectrometer system (OTT Parsivel Co., Germany) and a ground rain gauge on 9 September 2008, in Shanxi Province, China, was analysed. With the least squares method, spectrum data of Fenyang and Jiexiu stations were used to determine the first Z-I relationship Z_ott=159I_ott^1.5. With optimization Z-I relationships, the data of the ground rain gauge and radar reflectivity is used to calculate the second Z-I relationship Z_rad=200I_rain^1.2, the third is the WSR_88D default Z-I relationship Z_Def=300I^1.4. The precipitation process by using these three types of Z-I relationships was estimated, the accumulated rainfall was calculated, and the results of precipitation were compared with the ground rain gauge measured results. The results show that the precipitation estimation of the rain spectrum Z_ott-I_ott relationship had the best effect; using the rain gauge Z_rad-I_rain relationship to estimate precipitation provided the second-best effect; and the default Z_Def-I relationship calculation showed the worst results. The first method is nearly promoted 4% in contrast with the second method and nearly 18% in contrast with the third method.