The invention relates to a cathode side gas humidity adjusting system and method for a proton exchange membrane fuel cell, and the system comprises an air filter, an air compressor, an intercooler, anair humidifier, a fuel cell stack, an exhaust back pressure valve, a tail gas processor, a humidity regulator, a three-way valve, and a fuel cell control unit. An outlet of the fuel cell stack is connected with an inlet of the humidity regulator, and the humidity regulator is connected with the air humidifier. The air humidifier is connected with the three-way valve, and the fuel cell control unit is connected with a water content sensor and a humidity regulator in the fuel cell stack. The humidity of air entering the fuel cell stack can be automatically adjusted according to the actual working requirements of the fuel cell stack, the phenomenon that the interior of the stack is too dry or too wet is avoided, the water balance in the stack is maintained, the output performance of the fuelcell is effectively improved, the operation stability of the fuel cell is guaranteed, and the reliability and durability of the stack are improved.