Constant speed control method for vehicle and device thereof

A constant speed control method for a vehicle and a device thereof are disclosed. The constant speed control method includes steps of: firstly examining if a constant speed switch of the vehicle is pressed by an electronic control unit; allowing the electronic control unit to enter a constant speed mode when the constant speed switch is pressed; and sending a signal by the electronic control unit, so as to lock a throttle locking device for maintaining an opening degree of a throttle valve, and to adjust an transmission ratio of a ECVT for maintaining the constant speed driving of the vehicle. The present invention is further related to a constant speed control device for implementing the above method.