Ultrasonic power transfer using piezoelectric devices is a promising wireless power transfer technology for biomedical implants． The depth of implantation is the key parameter of ultrasonic power transmission system． To explore the effect of depth of implantation on the power transmission efficiency，an ultrasonic power transfer system is designed underwater． The depth of implantation is simulated by the distance between the transmitter and the receiver． The output voltage of the receiver is acquired at different distances firstly and then analyzed by SPSS and Matlab software with statistics method． The linear regression model is established on the basis of analysis． The model is verified by means of the ultrasonic power transfer system． The experimental results show that the ultrasonic power transmission efficiency is related to the distance between a transmitter and a receiver． The output voltage of receiver decreases with the increase of distance． The proposed model has good prediction effect with the fitting degree greater than 95 percent and the prediction error less than 10 percent．