Accuracy of nuclear power plant feedwater flow measurement is critical .Aiming at the impact of changes in orifice thickness parameters on the outflow coefficient of orifice valves in a nuclear power plant's feedwater measurement, computational fluid dynamics software was used to numerically simulate orifice valves with different structural parameters to obtain the differential pressure, outflow coefficient, and different Flow field distribution of the model. The results show that when the orifice plate thickness E does not change, the orifice thickness e changes from 6mm to 5mm, the orifice valve outflow coefficient decreases continuously, the values are 1.840 % and 2.125 %, respectively, and the actual measured flow value will be larger by the same percentage value; When the orifice plate e is kept unchanged, changing E from 11mm to 11.15mm, the orifice valve outflow coefficient becomes smaller, and the variation value is 1.56 %, and the actual measured flow rate will be 1.56 % larger.