Rapid growth in population and gross domestic product in Chinese urban regions such as Beijing has resulted in a large amount of CO₂ emissions from fossil fuels. Forests are considered the most important carbon sink to offset these carbon emissions. In this study we estimated forest carbon storage in Beijing using forest inventory data and the biomass expansion factors method. We analyzed trends in the forest ecosystem and its fractions along altitudinal gradients. We concluded that broad-leaved forest is the main forest carbon pool in Beijing, and forest carbon is mainly stored in plains at an altitude of <60 m a.s.l. and in the low mountainous area at an altitude of 60–600 m a.s.l. Forest ecosystem carbon density in Beijing increases with altitudinal gradients but slightly decreases in regions at altitudes that range from 200–400 m a.s.l. Forest vegetation carbon density of the plain area at an altitude of <60 m a.s.l. is much higher than other areas, attributable to the high ratios of the plantation of Populus spp. and Larix principisrupprechtii with higher vegetation carbon densities than others and human practices such as fertilization and irrigation which are beneficial for vegetation carbon accumulation. The forest soil carbon density in Beijing also increases with increasing altitude, attributable to decreasing soil carbon output with altitude. Management practices such as understory cultivation, irrigation, fertilization and scarification directly accelerate carbon emissions from soil heterotrophic respiration which decreases with altitude. Soil erosion in areas that range from 200–800 m a.s.l. also decrease with altitude as management practices decrease. This study will underpin scientific knowledge for local forest managers to adopt more appropriate practices to protect forest ecosystems and to improve forest carbon storage.