Fuel cell system and fuel supplementing method thereof

The invention provides a fuel cell system and a fuel supplementing method thereof. The fuel cell system comprises a fuel groove, a fuel feed unit, a cell module, a condensation recovery unit, a liquid level regulating unit, a detection unit and a control unit. The fuel feed unit supplements a first fuel for the fuel groove; the fuel groove provides a second fuel for the cell module which generates first gas; the condensation recovery unit condenses the first gas to recovery liquid; the liquid level regulating unit leads the recovery liquid to the fuel groove so as to control liquid level of the second fuel in the fuel groove to be fixed; the detection unit can detect a current signal provided by the cell module; and the control unit calculates magnitude of current according to the current signal, when the magnitude of current reaches a preset value, the first fuel is supplemented to the fuel groove from the fuel feed unit. The accurate supplement of the fuel can be achieved by the fuel cell system.