Using data from 6 sample plots and 36 sample trees in the Songnen Plain, northeastern China, we established a compatible biomass model of poplar, measured carbon content rate of each component of the plantations, estimated carbon content in each organ of poplar, and analyzed the characteristics of carbon content density of the ecological system of poplar. The results were as follows. Diameter at breast height (DBH) and forest age were the main factors that affect carbon content in various organs of poplar. Carbon content rate of each organ ranged between 0.4427 and 0.4848. Difference in carbon content rate of each layer of understory species was significant, with that in deadwood layer between 0.4568-0.4711, 0.3683-0.4454 in litter layer, 0.4184-0.4600 in half-decomposition layer, and 0.3506-0.3729 in herb layer. Biomass and carbon storage rate of 14-28-year-old poplars grew with forest age; the proportion of biomass and carbon storage of stem kept around 0.60, while those of canopy at 0.17. Carbon storages of 14-, 21- and 28-year-old poplar ecology systems were 230.3449,280.9064 and 356.4973t/ha, respectively, and carbon storage of each component in this ecology system was in the order as soil layer > vegetation layer > litter layer. Since the understory vegetation layer in this region consisted mainly of herbs, the proportion of carbon storage of tree layer was more than 99%. The soil layer was very thick, so the carbon storages of the 14-, 21- and 28-year-old poplar ecological system depended mainly on soil and grew with forest age, which were 216.5626, 262.3598 and 335.3581t/ha, and the proportion was all over 93%.