Abstract: When the results of the pump flow test of the low head safety injection system (LHSI) of CPR1000 do not satisfy the acceptance criteria, the throttle elements in the pipeline system need to be adjusted. The adjustment method of the lower limit flow orifice plate can be obtained through the calculation and analysis of the engineering fluid mechanics. By using this method, the size of the restricted throttle elements can be obtained accurately. The analysis method effectively solves the problem of how to change the throttle elements in the pipeline system when the low head safety injection pump flow test do satisfy the requirement of the criterion.