A new kind of inertial piezoelectric actuator for a micro in-pipe robot is proposed and studied. The actuator is composed of a body, corresponding to a mass rod, and four elastic legs. Each leg is a composite piezoelectric bimorph beam, made up of a middle metal element, an upper and lower piezoelectric elements. The mechanism is driven by an asymmetric waveform voltage, such as saw-toothed waveform, and utilizes the dynamic relationship between the maximum static friction force and the inertial force. To study the actuator, firstly, the constituent equation of a composite piezoelectric bimorph under both applied voltage and external force was inferred by thermodynamics. Secondly, the dynamic model of the actuator was established analyzing the relationship between the locomotive states, viz. displacement and velocity, and design parameters, such as piezoelectric strain constant, elastic modulus, length, width and thickness of the piezoelectric element, actuator mass, and driving voltage. At last, the dynamic equation was solved and the theoretical calculation of the inherent frequency was more consistent with the experimental data, which proved the rationality of the model. All these lay a theoretical foundation of the micro actuator parameter optimization and more research on a micro robot.