Abstract: A fuel cell system capable of improving the utilization of a mixed fuel includes a fuel cell, having an anode input terminal, a cathode input terminal, an anode output terminal and a cathode output terminal; a selective separator, having an input end, a hydrogen output end, and an unused gas output end; a hydrogen pump; a purge valve; and a steam trap. The fuel cell system can improve the separation efficiency of a hydrogen gas and reduce the hydrogen concentration of an exhaust gas to less than 4 vol %.