A layer built ultrasonic transducer based on PVDF piezoelectric film is proposed to solve one of the key technical issues concerning the low input/output gain in the piezoelectric film ultrasonic transducer. The amplitude frequency characteristics of ultrasonic transducer is educed by theory deduction and software simulation. Moreover the dependence of its gain, resonance frequency and bandwidth on the thickness and the layer number of the film could be reasonably caculated by the simulation result, which is very important for shortening the design cycle，reducing the product cost of ultrasonic transducer，and improving its conversion gain and sensitivity.