In the ultra-high water cut stage of fault block reservoirs, residual oil becomes dispersed, prompting the drilling of local infill wells to enhance oil recovery. Current methods for determining the location of these infill wells primarily focus on qualitative analysis of residual oil distribution using numerical simulation models, followed by manual well location design. This approach is limited as it fails to cover the entire optimal solution space and often overlooks economic constraints. This paper addresses these limitations by integrating both technical and economic factors to determine the optimal well location range in fault block reservoirs. The approach involves evaluating residual oil distribution and setting net present value (NPV) as the target function, with well location being the variable. A mathematical model is created for optimizing well placement, and the fish swarm algorithm is applied to find the optimal solution. This method allows for the optimization of well locations to maximize NPV. The proposed method has been applied to a fault block reservoir, resulting in a significant enhancement of oil recovery. The recovery rate of the target main reservoir increased from 27.6% before optimization to 32.86% after optimization. This approach eliminates the need for human intervention, improves work efficiency, and provides an effective method for optimizing the location of infill wells in fault block reservoirs.