Sun-photometer is one of the main instruments for ground-based detecting atmospheric optical parameter of whole atmospheric layer. The atmospheric optical parameters of lower atmosphere are more desiderated in some researches and projects. Based on the assumption of uniform parallel sphere of stratified atmosphere, a new algorithm for average extinction coefficient of stratified atmosphere measured by sun-photometer is put forward. When dealing with the data measured by sun-photometer POM-02, the relative error of atmospheric optical depth gained by stratified atmospheric algorithm and whole atmospheric layer algorithm is less than 3%. The atmospheric average extinction coefficient of lower atmosphere got by stratified atmospheric algorithm is compared with the atmospheric average extinction coefficient by lidar. The results show that they have a good consistency. Therefore, the stratified atmospheric algorithm is feasible, and it extends the sun-photometer′s application.