Abstract: In the 1000 MW nuclear power units FCB（fast cut back） test,the medium pressure control valve can not be closed in time,the deviation of the valve position feedback and instruction run over the limit ocurred,which caused the turbine trip protection action.Through the analysis,it was found that the mismatch of the aerodynamic moment and the spring moment was the cause of the problem.The three-dimensional flow field was numerically simulated by FLUENT,and the change regulation of aerodynamic moment was obtained.The conclusion shows that the aerodynamic moment first increased and then decreased with the increasing of opening degrees and the maximum value was reached when the opening degree was around 30°.At this point,if the aerodynamic moment did not match the spring moment,the medium pressure valve would close slowly and even clamped,leading to the deviation of the valve position feedback and instruction run over the limit.