Abstract: There is an experience feedback that a control valve choked flow problem in the steam generator blowdown system of Ling’ ao phase II nuclear power plant resulted in a frequency system trip. The problem is studied by using a method to calculate the choked flow in the control valve and orifice and some practical solutions are suggested to prevent the damage of choked flow. The control valve recovery facter FL and the pressure drop of orifice have a great influence to choked flow. To reduce the damage of choked flow,it is recommended to select a higher recovery facter FL control valve and install an orifice with bigger pressure drop downstream the control valve to decrease the pressure drop of control valve and increase the pressure downstream the control valve.