Abstract: It is difficult to identify the thrust due to the vortices generated by an undulating motion as well as the shape and frictional drag as fish swims forward and encounter. This is because that the direction of thrust and drag are opposite but in the same direction. In this study, we investigate the hydrodynamics of an undulating fish-like bionic structure model from the perspective of force decomposition and the associated force elements. Each bionic structure undergoes lateral motion in the form of a stream wise travelling wave. It’s shown that added-mass force is the most significant thrust source among all considered motions and arrangements. By identifying the variations of the vorticity thrust elements with various flow structures, we found that the vortices round the leeside of deformable body are beneficial for thrust generation; however, the average volume vorticity contribution is opposite to swim forward. These results lead to understand why inviscid theory proposed by Lighthill and Wu could be used to explain propulsion of fish.