Based on the data of airborne Ka-band precipitation cloud radar (KPR) and droplet measurement technologies (DMT) obtained during a weak precipitation process of stratiform cloud system on 17 November 2019, the radar reflectivity (Z_c) calculated with the detected cloud particle spectrum is compared with the KPR observation (Z_m), and the causes of the deviation are discussed. The results are as follows. (1) Inside the stratiform cloud, Z_c and Z_m are in good agreement with a deviation and root mean square error of 4.1 dBZ and 4.3 dBZ, respectively. (2) The deviation and root mean square error between Z_c and Z_m are larger in areas with larger variation of KPR’s radial velocity and velocity spectrum width, or in thin cloud area, but there is a good consistency in the variation trend of radar reflectivity. (3) The variation of cloud particle number concentration (N_c), cloud particle effective diameter and cloud liquid water content all have certain influence on the deviation between Z_c and Z_m, but the deviation is generally controlled within the range of ±10 dBZ. With the increase of N_c, the deviation between the two becomes smaller; when Nc is greater than 200 cm-3, the deviation greater than 10 dBZ is rare. Z_c is smaller than Z_m when the effective diameter is between 50 and 230 μm and the liquid water content is between 0.15 and 0.50 g·m^-3; Z_c is greater than Z_m when the effective diameter is greater than 230 μm and the liquid water content is greater than 0.50 g·m^-3.