Abstract: Traditional reliability assessment methods of power system are based on their respective voltage grade, the weak spot of their own system can be discovered in this way, but it cant reflect the interaction between transmission system and distribution system. Considering the effect of transmission system and substation main electrical wiring, a top-down approach about overall voltage level reliability assessment is proposed in this paper. Taking an actual grid as example, the result shows that reliability index which is calculated by proposed method can truly reflect the power supply reliability level. Through the comparison of users’ reliability index, the influence of different substation main electrical wiring is analyzed.