Abstract: An in-service high pressure safety injection system（JND） of a VVER nuclear power plant was found to be unqualified due to abnormal fluctuation in the low flow recirculating test.The cavitation of the money throttle orifice plate of the rain circulation pipe is the main cause of flow fluctuation. In this paper, the improved design and calculation process of recirculating orifice plate is introduced, including the calculation of orifice plate plow rate, pressure difference, series, aperture and thickness, etc., and how to control the pressure drop of the service orifice plate to avoid cavitation is discussed emphatically. This improvement has been implemented in nuclear power plants, and the problem of recycle flow fluctuation has been improved significantly.