A system for humidifying a vehicle that is a fuel cell electric vehicle includes a fuel cell stack for producing electrical energy through an electrochemical reaction of hydrogen and oxygen, a water supply tank for storing water generated during electricity generation in the fuel cell stack, a HVAC apparatus for humidifying an interior of the vehicle using the water supplied from the water supply tank, a heater for generating steam by heating the water supplied from the water supply tank, and supplying the steam to the HVAC apparatus, and a controller that determines an operation mode of the system based on at least one of a vehicle state, a HVAC apparatus state, or an indoor temperature of the vehicle, and controls at least one of the HVAC apparatus or the heater based on the determined operation mode to control indoor humidification of the vehicle.