A rotary piezo motor with adjustable preload function is designed to study the rotational motion of an Impact Drive Mechanism (IDM). The motion principles of this Rotary Impact Drive Mechanism (RIDM) are investigated, the influencing factors of driving signal, structural parameters as well as piezoelectric element on motion characteristics of the mechanism are analyzed. The research results reveal that the rotational velocity of this motor is in proportion to the frequency or the voltage of driving signal, and in inverse proportion to M/m (the ratio of the main body and the weight). The velocity decreases as the charge time increasing of piezo element. Furthermore, the results also show that the torque-speed property and forward-backward rotation of rotary motor are both different from that of traditional motor. The torque-speed curve of the rotary motor is a fold line,and the forward rotational velocity is larger than the backward rotation velocity all the time. When the frequency,voltage and the charge time are 1100 Hz,50 V and 70 μs,respectively, and M/m is equal to 8.9, rotational velocity is about 1.75°/s and the maximum torque is 0.13 N·m.These results reported here show that the rotary impact drive mechanism is a multivariable system, whose performance is decided by many factors.