Ultrasonic transducer is a device for energy conversion, which can achieve energy conversion between electric energy and ultrasonic energy. Sandwich piezoelectric ultrasonic transducer has been used widely in the field of power ultrasonic for its excellent performance. In the structural design of the ultrasonic transducer, the length of front and back cover is directly related to the performance of the transducer. In the process of solving the front cover length to obtain a complex of the implicit function, general this function is solved using the graphical method, and the solution accuracy is relatively low. In this paper, Newton iterative method is used to solve implicit function on the front cover. It can obtain a more precise length of the front cover than graphic method. So the coefficient of the extension in the maximum value of the velocity ratio can be calculated, and then the specific dimensions of the transducer are designed. In this paper, Matlab is used to program and calculate, and obtain the best result.