[Objective] The objective of this study is to explore the effects of long-term plantations on organic carbon pool in the saline-alkali soil of the Yellow River delta, in order to provide the scientific basis and technical reference for giving full play to its ecological functions and economic benefits.[Methods] Four pure plantations of Robinia pseudoacacia, Fraxinus chinensis, Ulmus pumila and Ailanthus altissima, and three mixed plantations were selected as the research objects. The change characteristics of active organic carbon and carbon pool management index in different plantations soil were studied. Standard pool was set and multipoint mixed sampling method was used.[Results] The microbial biomass carbon contents in three mixed plantations were significantly higher than that in the other forest land. The microbial biomass carbon content in R.pseudoacacia and F.chinensis mixed plantation was 26.16%, 17.62%, 88.41%, 47.99% and 21.64% higher than that in increases in wasteland, and three pure plantations of R.pseudoacacia, F.chinensis, U. pumila and A. altissima, respectively. In comparison with wasteland, the dissolved organic carbon(DOC) contents in different plantations increased greatly, while the DOC content was apparently higher in mixed plantation of R.pseudoacacia. and F.chinensis than that in other plantations. Compared with wasteland, the total organic carbon (TOC) contents in different plantations increased significantly, which were observably higher in mixed plantations than that in corresponding pure plantations. In addition, the variation trends of different active organic carbon contents in forest land soils were as follows:low-active organic carbon > mid-active organic carbon > high-active organic carbon. The mixed plantation of R.pseudoacacia. and F.chinensis. was beneficial to the accumulation of low-active organic carbon. Additionally, the soil carbon pool indexes of plantations increased significantly in comparison to the wasteland. The carbon pool management index in mixed plantation of R.pseudoacacia. and F.chinensis. was the highest, which was 46.51, 34.88, 27.55 and 33.38 higher than that in four pure plantations of R.pseudoacacia, F.chinensis, U. pumila and A. altissima, respectively. There were no significant differences among the mixed plantations of R.pseudoacacia. and U. pumila, and R.pseudoacacia. and A. altissima.[Conclusion] In comparison to pure plantations, the mixed plantations had better effects on soil organic carbon pool improvement, and were beneficial to soil fertility enhancement and promoted the growth of forest tree.