Abstract: An S-typed flexible fish is modelled with incompressible Neo-Hookean material to describe flexible deformation of the fish body in fluid-structure interaction (FSI), in which the fish has a self-propelled motion in the direction of movement. The immersed boundary method (IBM) and the finite element method (FEM) were used to model the self-propelled flexible deformation fish with fluid-structure interaction, considering the effects of internal forces resulting from fish muscles. The study shows that the swimming speed of the fish increases fluctuantly with the constant S-shaped swing of the fish body, and the swing patterns of the fish body are based on the hydrodynamic performance of the interaction between the fish and fluid, as well as the fish body stiffness. The vortexes are firstly formed around the head of the fish, then move towards fish tail along the fish body as the fish moves forward, and finally shed from the tail in an S-shaped formation. The S-shaped wake flow directly affects the stability of the fish body and the swimming speed.