The microgrids will break away from the main network and turn into an island operation mode when faults occur. In order to support the load with sufficient power to reduce the risk level, the Ordered Binary Decision Diagram (OBDD) is used to search the islanding and restoration strategies of distribution network. A four-phase method is proposed, in which the strategies can be equivalent to the on-off state of lines. The OBDD models, objective function of the network and the Boolean expressions of all constraints are established and solved to obtain the islanding and restoration strategies. Because the distribution network has a large number of components and high failure rate, a modified state sampling method is used to evaluate the risk level. Experimental results show that islanding and reconfiguration strategies can be searched efficiently and the risk level of the network is reduced using the proposed method. This work is supported by National Natural Science Foundation of China (No. 51707111).