Magnesium slag is a solid waste from the refining process of magnesium metal, facing problems such as low activity and poor volumetric stability. In this study, a CO2 sequestration fiberboard (CFB) was prepared with magnesium slag and pulp fiber as the raw materials, and the effects of autoclaving curing, carbonation curing and carbonation-autoclaving sequential curing on the properties of CFB and the interfacial microstructure between magnesium slag and pulp fibers were systematically investigated. The results show that the CFB after only autoclaving curing had poor volumetric stability and flexural strength of 4.5 MPa. While the flexural strength and CO2 uptake of CFB reached 17.3 MPa and 14.4% respectively after 8 hours of carbonation curing. Moreover, the flexural strength of CFB can be further increased to 20.2 MPa by carbonation-autoclaving sequential curing. The microstructure and nanoindentation results show that calcium carbonate crystals, including aragonite and calcite, together with C-S-H filled the pores and enhanced the interfacial micromechanical properties between the pulp fiber and the hardened paste, which is the main reason for the high flexural strength of CFB after carbonation-autoclave curing.