Distribution laws of the ghost reflection at the image plane of the mid-infrared transmission optical system with four lenses are researched through experiments. Research results show that with the changing of the incident light angle, both the ghost image center and the focus of the transmission light at the image plane shift relative to the center of field of view. While under the same conditions, the ghost image shows a larger displacement than the transmission focus. According to this imaging law, an effective method is carried out to suppress the ghost image. By tilting the main lens rising to ghost image with a certain angle, the ghost image will be removed from the field. The validity of this method is also verified by software simulation. The method provides an effective way to avoid ghost image in complicated optics system.