A method and device for vehicle speed control when towing a heavy load trailer are disclosed. The method includes determining whether a vehicle is in a trailer towing mode; determining whether the vehicle is in a slope-climbing situation or not using a G sensor based road gradient value when the vehicle is determined to be in the trailer towing mode; calculating a difference between the G sensor based road gradient value and a torque-based road gradient value, and determining whether a towed trailer is a heavy load trailer on the basis of the calculated difference, when it is determined the vehicle is in the slope-climbing situation; and performing shift control using a heavy load trailer dedicated shift map when the towed trailer is determined to be a heavy load trailer.