Several interferential images are shown in the experiment of measuring the focal length of thin convex lens by auto-collimation method, including the direct reflection imaging of the rear surface of convex lens. It is found that an isometrical and handstand interferential real image is obtained by adjusting the position of convex lens and plane mirror. However, the brightness of the interferential image is weaker than that of the image achieved by auto-collimation method. In this paper, the imaging principle of the interferential image is explained by analyzing the multiple imaging process.