Abstract: According to the Mie backscattering cross-section formula, three types of drop size distributions (DSD), i.e., the M-P distribution, the simulated convective precipitation DSD and the heavy rain observations by disdrometer, are used to calculate the sum of the diameter's sixth power of all raindrops (Zd), the equivalent reflectivity factors (Ze) of 3.2 cm, 5.5 cm and 10 cm radars, and their corresponding differences to analyze effects of the differences on quantitative precipitation estimation (QPE). The results show that as precipitation becomes stronger, the total number density of all raindrops (N), the number density of large raindrops and Zd all increase. As the number of large raindrops increases, for 3.2 cm wavelength radar, Ze is greater than Zd, and their difference increases with rainfall intensity. The maximum difference might exceed 2 dB. For 5.5 cm wavelength radar, Ze changes from less than Zd to greater than Zd as rainfall intensity increases; the difference varies from -1dB to 1 dB. For 10 cm wavelength radar, Ze is always less than Zd and their difference might reach to -1 dB. Hence, when large raindrops existing, the shorter the wavelength is, the less accurate the radar reflectivity factor is under the same raindrop size distribution, which directly results in inaccurate QPE.