Abstract: The ability of obstacle avoidance based on forward looking sonar is a basic requirement of AUV(autonomous underwater vehicle) during a long range travel in the unknown ocean environment.A real-time obstacle-avoiding expert system based on a multibeam forward looking sonar is presented.The specifications of sonar and the description of obstacles are introduced.Structure of the obstacle-avoiding expert system is given.The decision-making process of the obstacle-avoiding expert system is illustrated.Knowledge base of the expert system is designed based on the information of task execution.The inference engine is designed and implemented according to the images of sonar,which can send instructions of obstacle avoidance and path re-planning.Lake test validates the obstacle-avoiding expert system.