A method for synthesizing a lane image is proposed in the present application. This method includes the following steps. M continuous image frames are retrieved at a frame rate f from a video image capture device. A quantity N for image mapping is determined based on a dash length L of a dashed line and a distance S between two dashes of the dashed lines. A frame interval for mapping image frames is determined based on the dash length L, the distance S, the velocity v, and the frame rate f. At least N image frames are retrieved from the M continuous image frames at the frame interval. The at least N image frames are synthesized to obtain the lane image using an image synthesizing device.