When the laser is shot into two parallel plane mirrors, it will cause multiple reflections. The laser optical lever method based on this principle has the advantages of simple optical path and fast reading and so on. However, there is a discrepancy between the theoretical analysis and experimental process in the incoming (inverse) shooting point of the laser in the plane mirror. The difference increases with the increase of reflection times. Moreover, the initial incidence angle of the laser cannot be ignored in the case of  fewer reflection times, so the original theoretical formula needs to be modified to avoid the error of measuring small distance. We modify the original theoretical formula of the multi-reflection optical lever method in measuring linear expansion coefficient of metal. By analyzing the experimental data, the more accurate linear expansion coefficient of metal is obtained.