In recent decades converting agricultural land to plantation forestry has been one of the major land use/land cover changes in many countries, and such land use/land cover change can affect ecosystem C, N and P cycling. In this paper we investigated the effects of afforestation with poplar (Populus simonii) on the stock and distribution pattern of C, N and P in former agricultural lands. The sites selected were 5- and 10-year-old poplar plantations with adjacent agricultural lands as control. The results showed that: (1) total ecosystem C, N and P stocks decreased in the initial stages of plantation establishment but increased with the stand age and the proportion of biomass C, N and P in total ecosystem storage increased; (2) C, N and P stocks in leaf, branch, stem, root and forest floor in the 10-year-old plantations were significantly higher than in the 5-year-old plantations, and the distribution of C, N and P in different plant components were different between the 5- and 10-year-old plantations; (3) compared with the agricultural land, C, N and P stocks in the mineral soil decreased in the 5-year-old plantation at the different soil depths (0-10 cm、10-20 cm、20-40 cm and 40-60 cm) but increased in the10 year-old plantation. In addition, we found that soil C, N and P distribution patterns among the three ecosystems were different. Our results indicated that agricultural lands afforestation with poplar affected the stock and distribution pattern of C, N, P at Keerqin Sandy Lands.