The effects of MIE scattering and vertical motion, including updrafts and downdrafts, on number concentration (N), radar reflectivity (Z), Rain Rate (I), Liquid Water Content (LWC) retrieved from the Doppler spectral density observed by Micro Rain Radar (MRR) are discussed in this study using numerical simulation method. MIE scattering mainly affects 1.20-4.00 mm diameter droplets. The mean deviation caused by MIE scattering is 2.74 m^-3 mm^-1 for N, 1.47 dBZ for Z, 0.0061 mm h^-1 for I and 0.0004 g m^-3 for LWC. The retrieved diameter is increased by downdrafts and decreased by updrafts, while downdrafts have greater influences on the retrieved diameter than updrafts, and the influence in lower altitude is greater than that in higher altitude. For instance, when the droplet diameter is 2.67 mm and the velocity of downdraft is 2.00 m s^-1 at 300 m height, the relative bias of the theoretical inversion (8.07 mm) is close to 200%, which is close to MRR threshold for droplet diameter. Meanwhile, downdrafts make the Doppler spectral density broaden and move towards particles with larger diameter, whereas effects of updrafts are opposite. To analyze the reliability of MRR, MRR data are compared with the data from the synchronous observations of disdrometer THIES. A precipitation event observed by MRR and disdrometer occurred during 0100 UTC to 1200 UTC 1 April 2015 at Jinan, Shandong Province. Observations of this event are used to examine the MRR data quality. Radar reflectivity, rain rate, number concentration, and median volume diameter (D₀) observed by MRR at the third gate are compared with observations of disdrometer. The result indicates that Z, I, N, D₀ detected by MRR and disdrometer agree well with similar variations and changing tendencies. Deviations of Z, I, and D₀ are 1.19 dBZ, 0.34 mm h^-1, 0.36 mm, respectively. Nevertheless, the values of I retrieved by MRR are larger and the diameter is smaller than that of distrometer. These biases are caused by updrafts in addition to systematic biases of the two instruments and analytical methods. Therefore, this study has verified the reliability of Doppler spectral density observed by MRR and retrieval parameters.