This paper studied the effect of different coupling agents on ultrasonic detection of partial discharge. Water-based ultrasonic coupling agents, medical coupling agents and silica gel coupling materials were compared with commonly used high-vacuum silicone greases to analyse the different response by using different coupling materials to detect the ultrasonic signals with different frequency and amplitude. It is found that the ultrasonic sensor has a better response when applied to medical coupling agents under the frequency 20kHz and 80kHz, the frequency response is better when applied to water-based ultrasonic coupling agents under the frequency 20kHz, 40kHz and 80kHz,but the both two coupling agents have the disadvantage of low viscosity. The frequency response under 40 kHz of sensor is better than other frequencies when applied to silica gel, but this material has the difficulty to achieve full coupling. The high-vacuum silicone gel with higher viscosity is preferred in the consideration of different application scenarios in the field. The silicone gel with the thickness of 0.5mm is also well recommended when the sensor is fixed by an external magnetic base and meet the test accuracy.