The outliers in the original rainfall samples could affect the accuracy of the formula, and the Pearson -III theory frequency curve fitting required a large amount of work when deducing the rainstorm intensity formula. Therefore, several methods of removing outliers were introduced and the influence of the coefficient of variation (Cv) and the coefficient of skew (Cs) on the Pearson-III Distribution were evaluated. Meanwhile, formula of rainstorm intensity was calculated using Excel Solver Add-in to optimize the Pearson -III curve parameters by the least square method and Gauss-Newton method based on the rainstorm data of the total 21 years from 1993 to 2013 in Youyang County of Chongqing City, and the fitting accuracy and error were analyzed. This method could effectively avoid the subjectivity of empirical fitting by eye in the process of fitting Pearson-III Curve. Through comparison and analysis, the proposed method of calculating the general formula of rainstorm intensity can improve the convergence speed and the accuracy.