The onboard charger (OBC)/low-voltage DC-DC converter (LDC) integrated power inverter for electric vehicles comprises multiple electrical elements that can heat up, which can cause performance degradation and system instability issues in electric vehicles. To address this, a cooling system is included in the OBC/LDC integrated power inverter, which primarily uses water as a coolant. In this water cooling method, controlling the flow rate of water is critical for uniform cooling of the component. Thus, we propose an optimization method that helps determine the design variables to ensure uniform flow rate in each channel of the water-cooled system. The control variables for fluid-flux flow distribution optimization are selected by performing flow analysis for the initial design shape and analyzing their effects on fluid-flux flow distribution. For optimization analysis, the central composite design technique was applied; in addition, multi-response surface optimization using the same flow rate for each channel was performed. The optimization results were compared and verified using desirability functions based on the flow ratio of the cooling water channel, product function, and error function. Among single-response objective functions, the product function showed excellent performance. However, optimization using a multi-response objective function showed significantly higher prediction accuracy than the single-response function: using the optimized design obtained with the multi-response objective function improved the fluid-flux flow distribution uniformity by approximately 90% or more than the initial design.