The observed precipitation and high resolution grid data of precipitation estimated quantitatively with radar are analyzed and compared, and it is found that the ratios of the precipitation estimation with radar and observed precipitation obey normal distribution. Credibility of observed precipitation can be derived from above regularities, and moreover, the distortion of observed rainfall can be estimated by building a regression equation. The precipitation during Typhoon Fitow is analyzed and calculated, and results show that good effect is achieved in the quality control of the observed precipitation actually by using radar precipitation estimation. This topic is worth further study.