The ability of supercritical/subcritical carbon dioxide (SC/Sub-CO2) to catalyze the production of 5-hydroxymethylfurfural (5-HMF) from fructose was studied in this paper. The optimal conditions for the conversion in SC/Sub-CO2 systems were determined, and sugarcane bagasse (SCB) was pretreated with SC-CO2 using 1-butyl-3-methylimidazolium acetate ([Bmim]OAc). Structural analysis of SCB samples was done by Fourier transform infrared (FT-IR), X-ray Diffractometer (XRD), Thermal gravimetric (TG), Solid state nuclear magnetic resonance (C-13 NMR) and scanning electron microscope (SEM) analyses. The contents of sugars in the acid and enzymatic hydrolysates were measured. The results showed that the optimum fructose conversion conditions of the SC-CO2/[Bmim]Cl system were 9 MPa, 120 degrees C and 0.5 h and the Sub-CO2/H2O system were 6 MPa, 180 degrees C and 2 h. Besides, SC-CO2/[Bmim]OAc was the most effective pretreatment process, with the lignin and hemicellulose removal rates in SCB being 8.78 and 4.83% respectively. FT-IR, SEM, XRD and TG analysis indicated that the SC-CO2/[Bmim]OAc pretreatment changed surface morphology, increased crystallinity and decreased thermal stability of SCB. Moreover, the yield of reducing sugars obtained by enzymatic and acid hydrolysis increased by 12.57 and 22.04%, respectively. The energy balance showed that SC-CO2/[Bmim]OAc pretreatment was efficient to increase the energy output of SCB. (C) 2020 Elsevier Ltd. All rights reserved.