In order to quantitatively evaluate the effect of log-normal spectral distribution assumption on retrieval of cloud optical thickness (COT) and effective particle radius (Re), the reflectance of water clouds with log-normal spectrum and modified Gamma spectrum was simulated for the 2nd and 5th channels of the AGRI (Advanced Geosynchronous Radiation Imager) onboard FY-4A satellite by the RTTOV (Radiative Transfer for the TIROS Operational Vertical Sounder) model developed by the European Center for Medium-Range Numerical Weather Forecasts. The variation characteristics of water cloud reflectivity with COT and Re under these spectral distribution assumptions were analyzed. On this basis, the look-up tables of COT and Re under two kinds of spectral distribution conditions are established, and the influence of cloud droplet spectral distribution type on cloud parameter inversion results is quantitatively analyzed based on a primary convective cloud in summer of 2020. The results indicate that for the 2nd channel, reflectance difference under the assumption of two cloud droplet spectra is within 0.1%--2%, whereas for the 5th channel, the reflectance calculated with the modified Gamma cloud droplet spectrum is 10%--20% lower than that calculated with the log-normal cloud droplet spectrum. The retrival results show that the effective particle radius r_eff retrieved with the log-normal cloud droplet spectrum is larger than that retrieved with the modified Gamma cloud droplet spectrum. The former is concentrated in 15--35 μm, while the latter is within 10--30 μm. The differences of COT retrieved with the two cloud drop spectra are relatively small, ranging from -2% to 5%.