Partial discharge detection is one of the most important methods for transformer insulation diagnosis, and the sensor -externally -installed method is the most used ultrasonic detection method at present. In view of the low sensitivity of current external ultrasonic detection method, the feasibility of improving transmission intensity of ultrasonic signal of transformer partial discharge by acoustic impedance matching is studied. By theoretical analysis, it is found that the transmission coefficient of ultrasonic signal can be increased by adding matching layer between transformer oil and shell when the acoustic impedance of matching layer is between those of the transformer oil and shell. The variation rule of transmission coefficients with frequency and the relationship between center frequency and thickness of matching layer are obtained. The simulation test results show that when the acoustic impedance matching layer is added between transformer oil and shell, the amplitude of the measured ultrasonic signal is about double that of the ultrasonic signal measured in opposite side without the acoustic impedance matching layer, and the transmission coefficients of ultrasonic signal is significantly enhanced, which verifies the feasibility of improving the intensity of the measured ultrasonic signal by the acoustic impedance matching.