Abstract: The design programme of nitrogen gas cooling system of a plugging meter in the primaryauxiliary system of the First Fast Reactor(FFR) in our country is proved in the paper. Accordingto the design parameter as well as the working condition of the plugging meter, the temperaturecharacteristic curves of the nitrogen gas cooling system are given experimentally under the condi-tion simulating various maintaining powers of the plugging meter, then the heat exchange capability of the system is analysed further, and a result is presented that the nitrogen gas cooling sys-tem can guarantee the plugging meter in the FFR primary auxiliary system work normally in theprogramme-controlled mode.