It is very important and difficult to select initial parametric value and to evaluate the result which is applicable or not of nonlinear model. To solve nonlinear equation with Levenberg-marqurt iteration method, the traditional approach to select initial parametric value is based on the sense and multiple testing, and only focuses on the accuracy of the fitted value while ignores the applicability of the result theoretically. These shortcomings were overcome in this study through. In order to conquer these shortcomings, this paper makes the Nonlinear Regression Statistics and two key estimation indices β^T<1， β^N<1. The control function of β^T<1， β^N<1 in the various parametric states were analyzed and used to solve parameters of Weibull Distribution function as well. About 230 samples’datum of Moso bamboo (Phyllostachys pubescens) in Zhejiang Province were applied to study the distribution pattern of diameter of Moso bamboo forests. The results show that the diameter distribution pattern of Moso bamboo forest is conformed to Weibull Distribution through the P-P chart and Kolmogorov test. Using of the relationship with Weibull’s three parameters and characteristic factors of forests, two-dimensional distribution function is induced. In the presented report, we also demonstrated that two-dimensional distribution function had high accuracy in the measurement on the distribution patterns of diameter and age of Moso bamboo forests. Finally, the theoretical probability value of diameter and age distribution of Zhejiang Province were presented.