Abstract: According to the control rod hydraulic drive mechanism (CRHDM) and the characteristics of its hydraulic cylinder seal structure, the leaking ways of the hydraulic cylinder piston ring seal were analysed, its theoretical calculation model was built and its flow resistance formula based on the calculation model of differential pressure at the hydraulic cylinder piston ring seal was deduced. By analyzing the results of the CRHDM single cylinder experiment, constants in the formula of the flow resistance were obtained. Under the experimental condition, results from the formula and the experiment tally with each other. The formula and analysis would be helpful for the theoretical (analysis) and engineering design of the hydraulic (cylinder) piston ring seal structure.