Abstract: The motivation of this research is to explore the hydrodynamic interactions and swimming performance of fish school swimming. The paper used a sharp interface immersed boundary method based on virtual cells to numerically investigate the swimming of two fish with different lateral and longitudinal distances and phase differences of tail beats. The hydrodynamic performance and flow fields of the two fish were comprehensively analyzed. The research results indicate that, under appropriate positions and corresponding tailbeat phase differences, the trailing fish can significantly enhance its swimming performance, while also affecting the leading fish's swimming performance. This article suggests that the optimal choice for lateral spacing should not be less than 0.3 time of the body length. In addition, by considering the pressure in the flow field, the hydrodynamic interaction mechanism between the two fishes is understood. It is found that the generation of thrust between the two fishes is simultaneously influenced by both the body pressure and the wake-induced forces. Analysis shows that the dominance of these two influences will change with the position between the two fishes. When the streamwise distance is less than 0.4 time of the body length, it is dominated by the body pressure. When the streamwise distance is greater than 0.4 time of the body length, it switches to being dominated by the wake-induced forces. This study provides insights for the understanding of the collective swimming behavior and supports the development of biomimetic underwater vehicles.