Observation of rain drop size distribution (DSD) with a vertical pointing micro rain radar (MRR) has important significance in the precipitation measurement field. The Mie theory of large particle, vertical wind and air turbulence have great influences on the retrieval of DSD measured by MRR. This paper simulates the process of how three factors affect the inversion of DSD and the calculation of other precipitation parameters, then makes the errors analysis. The Mie theory of large particle is more suitable for the scattering of the precipitation particle than Rayleigh approximation, which can have a correction for the underestimation of number density of small droplets and the overestimation of that in middle field. The vertical wind results in inaccurate estimation of particle terminal velocity, so its diameter is measured with some errors by empirical relationship, which affects the calculating accuracy of radar reflectivity and rain rate. Air turbulence can broaden the power spectral density, of which the impact on the inversion results of DSD are concentrated in small droplets field. Then the measured data by MRR is analyzed and the results prove the impacts of those factors. Finally, according to the analysis and application limits, the prospect of the future research trend of particle size distribution is conducted.