The present disclosure provides a thermal-activated pressure relief device of a fuel cell vehicle, the thermal-activated pressure relief device including: a hollow body mounted at an outlet of a hydrogen tank; a hydrogen discharge block having a plurality of first hydrogen discharge holes and mounted and fixed in an upper portion of the hollow body; a hydrogen discharge pipe having a plurality of second hydrogen discharge holes and connected to a bottom of the hydrogen discharge block; a piston fitted on an outer side of the hydrogen discharge pipe to be able to move up and down and to open and close the second hydrogen discharge holes; a stopper mounted in a lower portion of the hollow body and limiting a downward movement distance of the piston; and a melting alloy disposed between the piston and the stopper and melting at a predetermined temperature or more.