The invention discloses a fuel cell auxiliary system and a fast cleaning method for cathodes during shutdown, comprising: an air filter, an air flow meter, an air compressor, an intercooler, an air bypass valve, a humidifier, and a stack air inlet temperature sensor, electric stack empty humidity sensor, air back pressure valve, electric stack, electric stack water outlet temperature sensor, water pump, electric three-way valve, radiator, voltage sensor, AC impedance, current sensor, auxiliary system and shutdown of the present invention The cathodic purging method requires few changes to the system, and the components are simple, which is favorable for integration. When the fuel cell is shut down, the battery stack can be quickly dried up to facilitate the next normal startup, and it can play an important role in the field of fuel cell vehicles.