Abstract: To obtain the index dispersion and further support the probability distribution, an efficient method is proposed for the moment and semi-invariant calculation in the reliability index. The method is based on the state enumeration and impacts increment method, improving the calculation efficiency of the moments of each order. Moreover, the relations between the origin moment, central moment and semi-invariant are utilized to realize the effective calculation of the moment and the invariant index. In the IEEE-39 system and IEEE-118 system, errors of the proposed method for calculating first-order to third-order moments are less than 1%, which saves half of the computing time compared with the Monte Carlo method. The method provides key parameters that support further analysis of the probability distribution.