This paper proposes a new reliability evaluation method for distribution system to solve the problem of repeatedly passing through the network topology. Element blocks and load blocks are established with switches as the boundary. Element level and switch hierarchy are proposed to determine the location of the protection elements, which can be combined with highly sparse equivalent branch connection matrix to fast analyze the accessibility between load and power or backup power to establish equivalent load failure result type list. Then the failure element is simulated in MCS and by analyzing which block it belongs to determine the type of load, the failure frequency and the failure duration time are counted to calculate reliability indices. The weak parts of load points and system are identified through the failure time and the failure power supply of related component. The reliability assessment results of the example show that the algorithm can correctly and efficiently calculate the reliability index and is suitable for complex distribution system reliability assessment.