Raindrop size distribution (DSD) plays an important role in microphysics of clouds and precipitation. A case study about a heavy rainfall with squall line characteristic passing through Beijing is presented by analyzing the DSD observation data. The variability of DSD and induced rainfall parameters in different rain regimes, namely initial convective rain, transition and trailing stratiform, is investigated. Compared with the transition and trailing stratiform region, leading convective line is characterized by large raindrop concentrations(>4600 counts/m3), large mass-weighted mean diameter(2.69 mm), and maximum drop size(6.3 mm). Convective and transition regime have the similar DSD with standard deviations(>0.3 mm), wider than stratiform region(>0.24 mm). The empirical Z-R relationships are conspicuously different between different rain phases.