Based on observations of Thies optical disdrometer and CINRAD/SA Doppler weather radar data, the evolution characteristics of the raindrop size distribution and integral parameters at different phases during a typical mid-latitude trailing stratiform squall line on 18 July 2017 are analyzed. The main results are listed as below. 1) The echoes of mature squall line include convective zone, transition zone, and trailing stratiform cloud zone. New convective zones are continuously generated in front of the convective zone and are integrated into the main convective zone, where strong reflectivity factors appear and multiple centers of great rainfall intensity are formed. 2) Crossing the convective zone, the increase/decrease phase of rainfall intensity witnesses fewer/more small particles (Diameter<1 mm) and extra large particles (Diameter>5 mm) as well as lower/higher raindrop concentration and reflectivity factor. During the intensification/weakening phase of the squall line, the raindrop size distribution has relatively large/small peak diameter (0.44 mm)/(0.19 mm) and there are more/fewer large (Diameter＞3 mm)and extra large particles. 3) The three parameters of Gamma distribution in the raindrop size distribution of convective zone, transition zone, and stratiform cloud—N0, μ, and λ, have obvious stratified characteristics as rainfall intensity increase. Under the same rainfall intensity, the three parameters of convective cloud and transition zone are larger than those of stratiform cloud; the λ-μ relationship has consistency for different phases of the squall line and different types of precipitation, and a quadratic polynomial can better fit the λ-μrelationship. 4) The distribution of normalized NW and D0 can be utilized to distinguish convective and stratiform cloud precipitation and a new equation of separation line is provided; furthermore, the characteristics of the raindrop size distribution are obviously different in the development and weakening phase of the squall line, which indicates the microphysical mechanism of precipitation has changed during the process. Cold cloud processes have significant influence in the earlier phase, while warm cloud processes play a dominant role in the later phase.