Brassinosteroids (BRs) are important hormones that play crucial roles in plant growth, reproduction, and responses to abiotic and biotic stresses. CYP85A1 is a castasterone (CS) synthase that catalyzes C-6 oxidation of 6-deoxocastasterone (6-deoxoCS) to CS, after which CS is converted into brassinolide (BL) in a reaction catalyzed by CYP85A2. Here, we report the functional characteristics of rice (Oryza sativa L.) OsCYP85A1. Constitutive expression of OsCYP85A1 driven by the cauliflower mosaic virus 35S promoter increased endogenous BR levels and significantly promoted growth and biomass production in three groups of transgenic Populus tomentosa lines. The plant height and stem diameter of the transgenic poplar plants were increased by 17.6% and 33.6%, respectively, in comparison with control plants. Simultaneously, we showed that expression of OsCYP85A1 enhanced xylem formation in transgenic poplar without affecting cell wall thickness or the composition of cellulose. Our findings suggest that OsCYP85A1 represents a potential target candidate gene for engineering fast-growing trees with improved wood production.