The invention is applicable to the field of a reactor control and protection system, and provides a method and device for reducing the measurement deviation of an ex-core measuring system of a pressurized water reactor. The method comprises the following steps: controlling a reactor core to continuously overheat so as to enable the temperature of the reactor core to be less than or equal to a specified temperature value before carrying out heat balance test on a testing instrument system, and maintaining the temperature until ending of the heat balance test of the testing instrument system; carrying out heat balance test on the testing instrument system, and obtaining the thermal power through the testing instrument system; obtaining the nuclear power measured by an RPN channel during theheat balance test of the testing instrument system; and calculating Gk parameters according to the thermal power and the nuclear power. The method and device increase the average temperature of a loopby overheating the reactor core, and then improve the thermal power of the reactor core; and the improvement of the thermal power of the reactor core is beneficial to solving the problem that as theactual adjustment amount of the deviation is less than expectation, and the actual confirmation and adjustment times of the Gk parameters are increased.