Based on airborne Ka-Band Precipitation Radar (KPR) and Droplet Measurement Technologies (DMT) data obtained during a weak stratiform cloud precipitation system on November 17, 2019, we calculated the radar reflectivity with the detected cloud particle spectrum, compared it with the KPR observation, and discussed the error mechanism. Results are as follows. (1) Inside the stratiform cloud, the calculated reflectance factor (DBZC) and the KPR measured value (DBZM)are in good agreement, with a mean relative error and root-mean-square error of 4.1 dBZ and 4.3dBZ, respectively. (2) In areas where KRP detected radial velocity changes and velocity spectral width are large, or in thin clouds, DBZC and DBZM are in good consistency despite their poor-fit relatively. (3) The error between DBZC and DBZM is affected by the changes in cloud particle number concentration (Nc), cloud particle effective diameter (ED), and liquid water content (LWC) in the cloud. However, the error value is generally controlled within the range of ± 10 dBZ. The relative error decreases with increasing Nc, with precious few errors greater than 10dBZ when Nc is greater than 200cm-3. The calculated value is smaller than the KPR observation in condition with ED between 50~230 μm and the LWC between 0.15~0.5g/m3. On the contrary, the calculated value is larger than is observation as ED bigger than 230 μm and the LWC higher than 0.5g/m3.