An experimental study on flowrate and stability of a type of control valve of 600MW supercritical steam-turbine was presented by measuring instruments of static, dynamic pressure and vibration in self-designed test rig. The investigation shows that flow coefficient is 30% up more than that of the control valve of GX-1 type used widely in domestic power plants now, as small-medium lifts. If the relative lift (h/D) is less than 20%, the valve can always work steadily in all the pressure ratios. When the h/D is between 20% to 24%, big vibration of valve stem occurs if the pressure ratio is between 0.7 to 0.8. When h/D is more than 25%, relatively great vibration happens in a wide range of pressure ratios of 0.4 to 0.85.