Substation as a connection hub between the different voltage levels, which bearing the important functions of energy exchange, and it is becoming a research hotspot to assess the risk of substation. Firstly, proposed a method using the reachability matrix of graph theory to recognize connectivity, which make the problem of risk assessment of substation converted into a problem to discern the connectivity of load point and power supply point; Secondly, using the Monte Carlo method to extract the substation system state which considering the randomness of substation event ,then get the reachability matrix by modified the system state of each adjacency matrix，which judge whether the substation will loss load or not; Finally, combine with risk theory calculate the risk of substation. Simulation results of example show the validity and efficiency of method.