The present application belongs to the field of reactor control and protection systems and provides a Gk parameter calibration method and device for an ex-core measuring system of a pressurized waterreactor. The method comprises the following steps: determining a Gk parameter according to a thermal power variable of a test instrument system; determining a Gk verification parameter according to the current of an ionization chamber of an RPN channel; and then comparing an absolute value of a difference value of the Gk parameter and the Gk verification parameter with a preset threshold value todetermine whether the Gk parameter is applicable or not. By determining whether the Gk parameter is applicable or not, the Gk parameter applicable system is stable in operation and the number of timesof adjustment is reduced.