A fuel reforming system includes an engine combusting reformed gas to generate mechanical power; an intake line connected with the engine to supply the reformed gas and air to the engine; an exhaust line connected with the engine to circulate exhaust gas exhausted from the engine; a fuel reformer provided at an exhaust gas recirculation (EGR) line diverging from the exhaust line, mixing the exhaust gas passing through the EGR line with fuel and reforming the fuel mixed with the exhaust gas; a water temperature controller (WTC) provided at the engine to control coolant cooling the engine; a radiator for radiating a portion of heat generated from the engine to atmosphere through the coolant; a temperature sensor provided at the EGR line at a front end of the fuel reformer and measuring temperature of the exhaust gas at the front end of the fuel reformer; a coolant passage provided to connect an exit of the engine, the fuel reformer, the radiator, and an entrance of the engine in series; and a coolant supply control valve for supplying the coolant into an inside of the fuel reformer according to engine driving condition and temperature of the exhaust gas.