During the intensive observation period of the Watershed Allied Telemetry Experimental Research (WATER), a total of 1074 raindrop size distribution were measured by the Parsivel disdrometer, the latest state-of-the-art optical laser instrument. Because of the limited observation data in Qinghai-Tibet Plateau, the modelling behaviour was not well done. We used raindrop size distributions to improve the rain rate estimator of meteorological radar in order to obtain many accurate rain rate data in this area. We got the relationship between the terminal velocity of the raindrop and the diameter (mm) of a raindrop: <i>v(D)</i> = 4.67<i>D</i><sup>0.53</sup>. Then four types of estimators for X-band polarimetric radar are examined. The simulation results show that the classical estimator <i>R</i> (<i>Z</i><sub>H</sub>) is most sensitive to variations in DSD and the estimator <i>R</i> (<i>K</i><sub>DP</sub>, <i>Z</i><sub>H</sub>, <i>Z</i><sub>DR</sub>) is the best estimator for estimating the rain rate. An X-band polarimetric radar (714XDP) is used for verifying these estimators. The lowest sensitivity of the rain rate estimator <i>R</i> (<i>K</i><sub>DP</sub>, <i>Z</i><sub>H</sub>, <i>Z</i><sub>DR</sub>) to variations in DSD can be explained by the following facts. The difference in the forward-scattering amplitudes at horizontal and vertical polarizations, which contributes <i>K</i><sub>DP</sub>, is proportional to the 3rd power of the drop diameter. On the other hand, the exponent of the backscatter cross-section, which contributes to <i>Z</i><sub>H</sub>, is proportional to the 6th power of the drop diameter. Because the rain rate <i>R</i> is proportional to the 3.57th power of the drop diameter, <i>K</i><sub>DP</sub> is less sensitive to DSD variations than <i>Z</i><sub>H</sub>.