This paper presents an innovative obstacle avoidance method based on fuzzy control technology and Potential field method for a miniaturized and low cost USV (Unmanned Surface Vehicle), which is designed for offshore water quality testing. The micron DST sonar, a miniaturized mechanically side-scan sonar, is adopted as environmental information collection sensor, for its flexible control mode and low cost. The use of MCU (Micro Controller Unit) and android System ensure a good real-time control. We conducted a series of experiments both in swimming pool and in lake, with stationary and motive obstacles. The result shows that the method presented is suitable for the USV.