NOVELTY - The system has a de-mineralizer (19) that is arranged in branch line (18) branched from a cooling water circulating line (13). A first port of three-way valve (15) is connected to radiator line (20) along which the cooling water passing through a radiator (12) is flowed. A second port of valve is connected to a bypass line (14) formed in front of an inlet of radiator, while third port is connected to side of fuel cell stack (11) of circulating line. A de-mineralizer line is connected to second port, such that the cooling water passing through de-mineralizer is flowed through.    USE - Thermal management system for fuel cell vehicle e.g. motor car such as passenger car including sports utility vehicle (SUV), bus, truck, various commercial vehicle, watercraft such as boat and ship, aircraft, and hybrid vehicle, electric car, plug-in hybrid electric vehicle, hydrogen-powered vehicle and other fuel vehicle.    ADVANTAGE - The output stability and the durability of the fuel cell stack are improved by increasing the vehicle current limiting time of arrival due to the high temperature cooling water at the time of the high output operation during a summer season. The durability of the de-mineralizer is improved by shortening the contact time of the ion resin with the high temperature cooling water by closing off the de-mineralizer loop during high output of the high temperature cooling water. The heat radiating performance is improved in the high output state by selectively reducing the flow rate which is bypassed to the de-mineralizer side in order to reduce the pump operation rate, so that the fuel efficiency of the vehicle is improved.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for a thermal management method for fuel cell vehicle.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic diagram of the thermal management system. (Drawing includes non-English language text)    Fuel cell stack (11)    Radiator (12)    Cooling water circulating line (13)    Bypass line (14)    Three-way valve (15)    Branch line (18)    De-mineralizer (19)    Radiator line (20)