Chinese forests, characterized by relatively young stand age, represent a significant biomass carbon (C) sink over the past several decades. Nevertheless, it is unclear how forest biomass C sequestration capacity in China will evolve as forest age, climate and atmospheric CO₂ concentration change continuously. Here, we present a semi-empirical model that incorporates forest age and climatic factors for each forest type to estimate the effects of forest age and climate change on total forest biomass, under three different scenarios based on the fifth phase of the Coupled Model Intercomparison Project (CMIP5). We estimate that age-related forest biomass C sequestration to be 6.69 Pg C (∼0.17 Pg C a⁻¹) from the 2000s to the 2040s. Climate change induces a rather weak increase in total forest biomass C sequestration (0.52–0.60 Pg C by the 2040s). We show that rising CO₂ concentrations could further increase the total forest biomass C sequestration by 1.68–3.12 Pg C in the 2040s across all three scenarios. Overall, the total forest biomass in China would increase by 8.89–10.37 Pg C by the end of 2040s. Our findings highlight the benefits of Chinese afforestation programs, continued climate change and increasing CO₂ concentration in sustaining the forest biomass C sink in the near future, and could therefore be useful for designing more realistic climate change mitigation policies such as continuous forestation programs and careful choice of tree species.