Fuel cell system and control method thereof

The invention belongs to the technical field of fuel cells, and discloses a fuel cell system and a control method thereof, the fuel cell system comprises a superior stack, a subordinate stack, a partial oxidation reformer and a water vapor oxidation reformer. The upper-stage electric pile and the lower-stage electric pile are connected in series. The partial oxidation reformer is arranged at the upstream of the upper-stage electric pile and is configured to reform fuel gas of the upper-stage electric pile to form upper-stage anode reaction gas, the upper-stage anode reaction gas can be introduced into the upper-stage electric pile, and at least part of upper-stage cathode exhaust gas can be circulated to the partial oxidation reformer. The water vapor oxidation reformer is arranged at the upstream of the lower-stage electric pile and the downstream of the upper-stage electric pile and is configured to reform lower-stage electric pile fuel gas and upper-stage anode exhaust gas to form lower-stage anode reactio