Along with the development of research and application of the equipment state maintenance theory， an improved four-state component model is proposed. The abnormal state and maintainable state of components are redefined and the component status maintenance state and scheduled maintenance state are incorporated in the maintenance state while the conversion relationship between different states and the parameter calculation are determined. The components combining and subzoning is used to simplify the main electrical scheme to form a simple directional network， and the improved four-state component model is applied in the subzone reliability parameter determination. The least path matrix searching method is simplified by matrix multiplication to search the least path matrix for the directional network， with each fault state enumerated by the analytical method. The reliability index of the network is programmed and worked out. This method has been applied in the main electrical scheme in Heping±800 kV converter station with the correctness of the method verified by the results.