Abstract: In the nuclear power plant, the flow resistance is an important factor for the design of the primary system. This paper clarified the cause for the shape flow resistance by analyzing the sudden expansion flow. The effect of the downstream resistance object on the total resistance was also examed by analyzing the flow resistance of the sudden expansion configuration close to a uniform pore plate or a sudden contraction configuration. The designers were advised to analyze the coupling effect of the upstream and downstream resistance objects in detail. To a specific problem, special analysis and demonstration were necessary to make sure whether the total resistance was equal to the sum of the resistance of each object or not.