The present invention discloses a composite-image parking-assistant system, which is installed in a vehicle. When a driver drives his vehicle to passes at least one parking space, the system of the present invention uses camera devices to capture images involving the parking space. A processing unit converts the images into bird's eye view images and integrates the bird's eye view images into a composite bird's eye view surrounding map via the common characteristics thereof. A display device presents the surrounding map to the driver. The processing unit adjusts the coverage the vision field of the surrounding map according the relative position of the vehicle and the parking space to make the magnification of the vision field inverse proportional to the relative position. Thereby, the driver can park his vehicle efficiently and avoid collision.