In order to achieve controllable behavior in soft robots, it is necessary to analyze the dynamic response characteristics of soft actuators and thereby gain a deeper understanding of the dynamic response mechanisms of actuators. In this study, the two-way fluid structural interaction (FSI) method was used for the first time to reveal the transient flow field inside the soft pneumatic actuator and its influences on the dynamic motions under high-speed pressurization. Moreover, the simulation results were verified by experimentally by using a high-speed camera to capture the transient morphology of the actuator. The simulation results indicated that streamlines inside the actuator were not uniformly distributed, and the air pressure was not even either. Driven by the high-speed air flow, the head of actuator bent at first, resulting in the expansion of the chambers gradually, followed by the arching of the actuator and more violent variation in air streamlines and pressure distribution. Accompanying the deformation of the actuator, the stress distribution, as well as the streamline and pressure, changed dramatically within a total 0.1 second. Experimentally, the transient morphologies captured by the high-speed camera at various moments coincided with the simulation results. Compared to the conventional solid-mechanics finite element analysis (FEA) method, the FSI simulation showed better precision and accuracy. This work serves as a valuable reference for research on the fine-structured design and controllable motion of soft actuators and soft robots.