The invention relates to a humidification system and method for a fuel cell, and a fuel cell system. The humidification system comprises a voltage and current sensor, which is connected with the poweroutput end of a fuel cell stack and used for collecting the output current and the output voltage of the fuel cell stack; an air flow meter, which is arranged on an air inlet pipeline of an air supply system of a fuel cell stack system and used for collecting air flow signals entering the fuel cell stack; an exhaust tail gas circulation valve, with one end connected with the outlet of the air supply system of the fuel cell stack system and the other end connected with the air inlet of an air compressor of the air inlet pipeline; and a humidity controller, which is separately connected with the voltage and current sensor, the air flow meter and the exhaust tail gas circulation valve and used for estimating the internal resistance of the fuel cell stack according to the output current and the output voltage of the fuel cell stack and adjusting the openness of the exhaust tail gas circulation valve according to both the internal resistance of the fuel cell stack and the flow of air entering the fuel cell stack. Compared with the prior art, the humidification system of the invention has the advantages of high integration level, low cost, small size and the like.