Abstract: While the pectoral fins of the pectoral fin propelled robot fish flapping for propulsion, they also bring a longitudinal disturbance. That may cause the pitching moment of the body, and affect the usage of the robot. A pitching stability model of the pectoral fin propelled robot fish is built based on the conception of the traditional submarine pitching stability. The pitching system of the robot fish is proved to be a second-order linear system. By analyses, it is proved that the disturbance generated by the pectoral fins contains a constant component, one-time flapping-frequency component and two-time flapping-frequency component. Finally, a prototype swimming experiment is conducted. The experiment results indirectly prove that the pitching movement model and the flapping fins disturbance model are correct.