A method for warning a back side of a vehicle provided with a rear bumper multi carrier may include determining a carrier position by determining it is in a stowed state or drawing out state of a rear bumper multi carrier when a signal Reverse (R) step of a shift lever is produced, controlling ON or OFF operation of a detector of a rear side obstacle according to the stowed state or drawing out state of the carrier, and producing a warning signal when the rear side obstacle is detected in the detector operation controlling step.