Abstract: We studied the effect of the brassinosteroid biosynthetic enzyme DAS5 on growth, biomass and drought resistance of Populus hopeiensis. The ground diameter, root length, petiole length, leaf length, dry weight of shoot and root, ratio of root to shoot of transgenic P. hopeiensis plants were bigger and higher than those of non-transgenic plants. Under drought stress, dehydration and chlorina were slower in leaves of transgenic than wild-type seedlings, and transgenic seedlings showed stronger recovery after rehydration, with more new and well-growing buds. The relative growth rate was higher for transgenic than wild-type seedlings under water-controlled conditions. Soluble sugar and proline content as well as catalase and superoxide dismutase activity were significantly higher in transgenic than non-transgenic plants as drought conditions worsened. Thus, the growth of the transgenic plants and their resistance to drought stress was better than the wild types. The brassinosteroid biosynthetic enzyme DAS5 from Arabidopsis could promote plant growth and play an important role in resistance against drought stress of P. hopeiensis.