Abstract: A detecting method is presented for the robot with ultrasonic sensor in unstructured working environment. Setting up a structure model of the detecting system and describing its operation principle, the causes of detecting errors are analyzed. There are dynamic errors of the ultrasonic sensor due to its moving in the whole detecting process, as well as other errors caused by its ambient temperature and patulous angle. An integrated error of the detecting system is thus formed in unstructured environment. Analyzing the relationship among the integrated error and robot's moving parameters and structure parameters, the way to compensate the integrated error is presented, which provides a theoretical basis to improve the detecting precision of the system. Simulation results verified the necessity of such error compensation.