Abstract: It is of great value to detect Partial Discharge (PD) in transformer insulation sensitively on-line, to discover insulation defect in time, and to ensure the safety of the whole power system. In this paper, the ultrasonic attenuation, propagation velocity and waveform changes of PD is tested in oil-paper insulation through oil, insulation paper, steel, paper-steel composite medium and copper wire by using fiber-optic ultrasonic sensor. The results show that signal intensity of ultrasonic wave in oil, insulation paper, steel and composite medium can be attenuated, while the wave shape doesnt change. It also can be seen from results that the influence of copper wire is the most serious, which can not only decrease the intensity and speed of ultrasonic wave, but also distort the ultrasonic wave. In the end, the actual measurement is carried out in the solid transformer, which obtains better detection effect.