A device for reinforcing vehicle fuel tank improves a structure of a rigid reinforcement pillar mounted in an enclosed fuel tank by maintaining stiffness of the reinforcement pillar in a state in which the fuel tank is under positive pressure and an engine negative pressure acts thereon as well as inducing breakage of the reinforcement pillar when a large impact applies, such as in the event of vehicle collision, thereby preventing a fused portion between upper and lower plates of the fuel tank from rupturing.