The phenotypic characters and leaf micromorphology of Acer stenolobum and its related species, A. stenolobum var. megalophyllum and A. pilosum, were studied in order to explore the position of A. stenolobum in plant taxonomy. The results showed that: (1) the variation of phenotypic traits of A. stenolobum from different habitats mainly came from within the population. (2) The phenotypic traits of A. stenolobum and its related species collected from the same origin are quite different. (3) The average coefficients of variation reflecting leaf size and shape of key fruit indicators were relatively large, and unable to accurately reflect interspecific relationships. Therefore, it does not support the main classification basis of “Flora of China”, Flora of China and various local floras with leaf size and doublewing angle as the main classification basis for A. stenolobum, A. stenolobum var. megalophyllum and A. pilosum. (4) The epidermis and the lower epidermis of the leaves of A. pilosum have surface fur, and it does not fall off during the life cycle. It is slightly different from the records of “Flora of China” and Flora of China to the A. pilosum. According to the relationship between the plant leaf veins relationship by Xu Bingqiang, the characteristics of the leaf vein of A. stenolobum, and A. stenolobum var. megalophyllum are basically the same, but the A. pilosum is more primitive than the A. stenolobum, and A. stenolobum var. megalophyllum. Our research provided evidence for treat A. stenolobum as a variety of the A. pilosum in Flora of China.