Abstract: The operation characteristics of electromotor are simulated with RETRAN-02 when the electric network frequency falls based on the electro mechanics principle.The hydraulics characteristics of CPR1000 reactor main coolant pump and the resistance characteristics of the reactor coolant system are also simulated.The operation transient of the reactor coolant pump and electromotor is studied in detail when the electric network frequency falls.In the end,the transient behavior of CPR1000 reactor coolant pump and electromotor is analyzed when the electric network frequency falls at the rate of 4 Hz/s.