The invention provides a piezoelectric ultrasonic transducer and a preparation method thereof. The piezoelectric ultrasonic transducer comprises a vibrating diaphragm and a piezoelectric diaphragm arranged on the vibrating diaphragm in an attached mode. A resistor structure capable of generating joule heat to enable the local temperature of the vibrating diaphragm to rise under the power-on state is arranged on a marginal area, close to a base, of the vibrating diaphragm. Because the vibrating diaphragm adopts materials of silicon dioxide and the like, wherein the Young modulus of the materials is reduced along with rising of the temperature, the integral elastic coefficient of the vibrating diaphragm can be reduced by heating, the electromechanical coupling factor of the transducer is increased, and sound pressure output is changed; and on the other hand, the resonant frequency when the vibrating diaphragm vibrates is reduced along with reducing of the elastic coefficient, and dynamic adjustment of an ultrasonic wave output frequency is achieved.