In order to detect the internal defects of the workpiece more accurately, an automated ultrasonic test system for truss robot is proposed, and the power spectrum analysis technology is used to analyze the defect echo signal. The control system was developed and the ultrasonic testing software program was compiled, the control system composed of industrial computer and PLC can control the truss robot to realize the threedimensional movement, while driving the ultrasonic probe can test the inner defects automatically. Compared with traditional ultrasonic testing methods, the power spectrum analysis method that analyze the ultrasonic echo signal of the inner defects can obtain obvious different defect characteristics, and identify small internal defects more accurately. In consequently, this test and control system for the truss robot improve the accuracy and efficiency of the ultrasonic test.