A numerical calculation of Gamma distribution value was presented. Taking common two-parameter gamma distribution in hydrological analysis as example, employing the advantage of high accuracy calculation of Mathematica, the paper calculated the recurrence relation coefficients of non-classical orthogonal polynomials of weight function and further computed hydrological probability distribution. The results show that the method of probability distribution calculation has high accuracy, and is a general method for numerical integration of weight function and interval and can provide a way for the hydrological probability distribution calculation.