A great deal of attention has been paid on developing plant-derived hard carbon (HC) materials as anodes for sodium-ion batteries (SIBs). So far, the regulation of HC has been handicapped by the well-known ambiguity of Na+ storage mechanism, which fails to differentiate the Na+ adsorption and Na+ insertion, and their relationship with the size of d-interlayer spacing and structural porosity. Herein, bagasse-derived HC materials have been synthesized through a combination of pyrolysis treatment and microwave activation. The combined protocol has enabled to synergistically control the d-interlayer spacing and porosity. Specifically, the microwave activation has created slit pores into HC and these pores allow for an enhanced Na+ adsorption with an increased sloping capacity, establishing a strong correlation between the porosity and sloping capacity. Meanwhile, the pyrolysis treatment promotes the graphitization and it contributes to an intensified Na+ insertion with an increased plateau capacity, proving that the plateau capacity is largely contributed by the Na+ insertion between interlayers. Therefore, the structural regulation of bagasse-derived HC has provided a proof on positively explaining the Na+ storage with HC materials. The structural changes in the pore size distribution, specific surface area, d-interlayer spacing, and the electrochemical properties have been comprehensively characterized, all supporting our understanding of Na+ storage mechanism. As a result, the HC sample with an optimized d-interlayer spacing and porosity has delivered an improved reversible capacity of 323.6 mAh g(-1) at 50 mA g(-1). This work provides an understanding of Na+ storage mechanism and insights on enhancing the sloping/plateau capacity by rationally regulating the graphitization and porosity of HC materials for advanced SIBs. (C) 2020 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by ELSEVIER B.V. and Science Press. All rights reserved.