A vehicle door opening warning system including a control unit, a projection unit and a detection unit is provided. The control unit is disposed at a door of a vehicle. The projection unit is disposed at the door and is electrically coupled to the control unit. The detection unit is disposed outside of the vehicle and is electrically coupled to the control unit. When the detection unit detects a moving object existing within 5 to 30 meters of the vehicle, the detection unit produces a signal. The control unit receives the signal and controls the projection unit to project a warning message according to the signal. A vehicle door opening warning method is also provided.