Since the fluid-structure analysis is widely applied to assess the fluid flow state of electromechanical products, the design of fluid-structure becomes crucial in improving product performance and efficiency. In this study, a numerical prediction method for fluid-structure parameters is proposed to evaluate and optimize the flow state and quality of the flow field in a rectangular duct system. The modular design concept is adopted in this research, where the overall flow of the duct is considered as the sum of local flow fields within the duct modules. By analyzing the structural parameters such as cross-section ratio, pipe length, and flow direction within each duct module, a numerical prediction model for flow based on fluid-structure parameters is developed using numerical fitting techniques. Experimental results show that the flow rate prediction accuracy for normal modules is within 10%, and the flow rate prediction accuracy for modules with irregular structures is within 15%. It validates that this numerical prediction method, based on the flow structure, is a reliable tool for assessing and optimizing the flow state and quality of the flow field in the entire rectangular piping system.