As a new type of underwater autonomous vehicle, underwater bionic robot fish not only has excellent swimming performance and flexibility of fish, but also has the characteristics of simple structure, low movement noise and weak impact on underwater environment, it gradually become an important carrier for exploring underwater environment. In order to meet the needs of underwater exploration, this paper first introduces a bionic underwater robotic fish based on pneumatic driving. The robotic fish is composed of caudal fin actuator, rod, shell, tail fin and so on. The tail fin actuator is mainly composed of a soft airbag and a piston. The soft airbag changes its volume by receiving or discharging compressed gas, thereby push the piston rod to generate thrust and drive the tail fin to swing, thus realizing various underwater movements of the robotic fish. Finally, we constructs an experimental system to measure the swing angle of the tail fin of bionic robot fish, and the relationship between the swing angle of the tail fin and the input air pressure has been obtained, which enables the accurate control of the tail fin swing angle.