Green stem photosynthesis is well known, but the effect of stem photosynthesis on plant growth and development remains uncertain. In this study, green stems of semiannual Rosa chinensis were subjected to a shading treatment to explore the effect of stem photosynthesis on the growth and biomass accumulation. The results showed that (1) Stem photosynthesis affected biomass accumulation and allocation in R. chinensis. The stem shading treatment decreased the biomass of R. chinensis by approximately 18.7%. The proportion of biomass allocated to the stems increased while the proportion of biomass allocated to the leaves and roots decreased. (2) Stem photosynthesis facilitates an increase in nonstructural carbohydrate concentration, chlorophyll a concentration and chlorophyll a/b of R. chinensis. (3) The net photosynthesis in the stems of R. chinensis was negative, and biomass accumulation was significantly positively correlated with the dark respiration rate in stems, indicating that stem photosynthesis fixed CO₂ released internally by respiration. This study reveals that stem photosynthesis in R. chinensis enhances biomass accumulation by promoting chlorophyll fluorescence and dark respiration rates in the stems. Furthermore, stem photosynthesis contributes to the balanced allocation of biomass by enhancing the proportional distribution of biomass to leaves and roots.