In order to accurately valuate the running state of the PV array, based on the PV array radial distribution electric network structure and the characteristics of randomness of output power, this paper proposes evaluation method of PV array of running status based on layered and divisional forward and backward algorithm. It discusses mainly the layered and divisional forward and backward algorithm, how to search the branch and node, probability and statistics method and evaluation process. It refers to 5 MWp PV array parameters of the new 20 MWp PV station and uses Matlab7.0 to verify this method. Simulation results show this method can correctly output evaluation data probability distribution and probability density of 5 MWp PV array running state, and can accurately evaluate the running state of PV array. This method solves the problem that single algorithm accurately assess running status of large scale PV arrays. In addition, this method has less number of iterations, fast speed, and high computational efficiency.