A head up display system for a vehicle is provided, including an eye tracking device and a head up display device. The eye tracking device is configured to pre-locate an initial gaze position of a driver of the vehicle before the vehicle starts moving and to detect a gaze position of the driver in real time. The head up display device includes a projector and a processor. The processor is coupled to the eye tracking device and the projector. The processor compares the gaze position with the initial gaze position, and controls the projector to project a display image to a first projection position or a second projection position according to a comparison result, wherein the first projection position corresponds to the initial gaze position and the second projection position corresponds to the gaze position. A control method of a head up display system is also provided.