To improve the positional accuracy of the mobile robot at the worksite, the effect of ultrasonic sensor beam angle is analyzed on the basis of considering the incidence angle, and the functional relationship expression of ultrasonic distance measurement is established on regarding the ultrasonic sensor beam angle and the incident angle at the same time. Calibration method of the ultrasonic beam angle is proposed, and the ultrasonic sensor beam angle is obtained through the calibration experiment. It is clearly that the established functional relationship expression of ultrasonic sensor measurement improves the distance measurement precision evidently in the experiment. Based on the proposed expression, three positioning (lateral positioning, front positioning and orientation positioning) methods of the mobile robot are adopted by using two ultrasonic sensor installed on these any single side of robot at the worksite. Finally the method is used in the positioning experiment on the independentlydeveloped mobile robot, and the correctness and effectiveness of the proposed method are verified.