•Branch size of Betula alnoides was significantly reduced in the mixture.•Mixtures significantly increased the frequency of small branches.•Branch distribution was less affected by mixture.•Mixture of B. alnoides and Erythrophleum fordii was beneficial for both species. The quantitative and morphological characteristics of branches are key factors influencing both tree growth and wood quality. The effects of stand density and artificial pruning on branch development have been widely studied in monoculture, while branch development in mixed forests has rarely been reported. In this study, branch development of Betula alnoides, Erythrophleum fordii and Pinus kesiya var. langbianensis were evaluated in a 6-year-old mixed-species trial plantation in Yunnan Province, China, which included five treatments: monocultures of B. alnoides, E. fordii and P. kesiya var. langbianensis, and mixtures of B. alnoides with E. fordii or P. kesiya var. langbianensis. Branch quantity (number, proportion, and density), morphology (diameter, length, and angle), and distribution (height and orientation) were measured from 63 dominant or codominant trees randomly sampled in the trial plantation. The results showed that the number of live and total branches of B. alnoides in the mixture was significantly higher than those in its monoculture, and the branch density of B. alnoides in the mixture with P. kesiya var. langbianensis was also higher than those in its monoculture and the mixture with E. fordii. Mixing with B. alnoides had no significant effect on the branch quantity of P. kesiya var. langbianensis, but significantly lowered the branch quantity of E. fordii. The branch angle was not significantly affected by the mixture for these species. More small branches (branch diameter < 15 mm) were observed in the mixed plantations than in the monocultures for the three species, while significantly lower branch diameter and length were seen in the mixture than in the monoculture only for B. alnoides. Significant differences were absent in the branch distribution of B. alnoides and E. fordii among the four cardinal orientations regardless of whether they were in the monoculture or in the mixture. The variations in branch quantity and morphology along the stem were much more determined by species-specific attributes and less affected by the mixture. It was concluded that a mixture of B. alnoides and E. fordii could significantly control the branch size of B. alnoides, reduce both the branch size and branch quantity of E. fordii, and thus be beneficial for high-quality timber production.