Abstract: In this paper, the Jarratt iteration method was applied to solve the distribution function of P-Ⅲ, and the accuracy, error, computation efficiency and validity of the solution were discussed. The program was compiled with the C++ language; what' s more, it was proved with an example that it had high efficiency. Finally, the Jarratt iterative method was coupled with the dichotomy to solve the problem of iterative non-convergence. Jarratt iterative method can be applied into hydrological statistics in order to improve the ability of hydrologic analysis and forecast. Additionally, it can be used for better flood management and drought relief work.