The maximum precipitation data in Shanxi Province are collected from 107 meteorological stations from 1981 to 2012. In reference to those data, the i-t-p relationship among precipitation intensity, duration and recurrence period is derived by the method of Gumbel adjusting frequency. Based on i-t-p relationship, the parameters of rainstorm intensity formula can be extracted by the unconstrained nonlinear optimization method. A novel group parameter extracting method is tried. The spatial distribution of the parameters of rainstorm intensity formula is given in this paper. The results can be used to construct rainstorm intensity formula in areas without meteorological observation stations according to the characteristics of the underlying surface and climate zone.