An ultrasonic testing device that can make a robotic testing system reach the surface of a complex curved composite workpiece that is not easy to reach and perform a quality testing. By pumping a coupling liquid into the device so that the coupling liquid enters a waveguide and jets onto the surface of the workpiece, an ultrasonic wave can be transmitted in the waveguide and reach the surface of the workpiece and penetrate the workpiece, thereby achieving the purpose of quality testing of the workpiece. By providing two ultrasonic testing devices without a waveguide on both sides of a tested workpiece, respectively, and by mounting the waveguide on one side or both sides of the ultrasonic testing devices, it is possible to transmit the ultrasonic waves to the surface of the workpiece or to receive the ultrasonic waves from the surface of the workpiece.