Abstract: This research adopts a method of combining immersed boundary（IB） with fluid-structure interaction（FSI） method and the large eddy simulation（LES）, to simulate the internal flow of an axial pump and the fish movement in the pump passage for a refined analysis on the characteristics of fish motions in hydraulic machinery. The results show that the calculated head coefficient has a deviation of only 2.7% against the experimental data under the design condition at the same Reynolds number, verifying the feasibility of this new method for the pump flow simulations. Compared with the traditional methods where fishes are simplified into flow tracers, the method adopted in this study is capable in capturing the complexity of fish trajectories, sudden changes in fish orientation after hitting the blade, and the vortex formation, shedding and structure in the fish wake, thereby helping to obtain accurate assessment of fish injury and design optimization of fish friendly hydraulic machinery.