Ti₃C₂T_x MXene with two-dimensional (2D)-layered structure shows potential application in various fields owing to its superb metallic conductivity and excellent solution processability. However, a fundamental challenge limiting the implementation of Ti₃C₂T_x in practical applications is its susceptibility to oxidation in either humid or aqueous environments only within a few days, which results in the disassembly of the 2D-layered structure and severely deteriorates the functional properties including electrical conductivity, mechanical strength and EMI shielding performance. Herein, we firstly demonstrate a robust procedure to protect sensitive Ti₃C₂T_x from degradation by modification with imidazolium-based inion liquid (IL), which remarkably improves the chemical stability of Ti₃C₂T_x in aqueous and significantly increases the mechanical strength of assembled freestanding Ti₃C₂T_x film (IL-MXene). The crystalline structure of IL-MXene sheet remains intact up to 30 days and preserves the 2D-layered structure as long as 8 months in aqueous, meanwhile, the tensile strength of freestanding IL-MXene film is as high as 75.9 ± 4.9 MPa and the increment is up to 84% when compared to that of untreated MXene film (41.2 ± 4.5 MPa). A novel mechanism for improved chemical stability of MXene sheet was proposed and elucidated based on the radical-scavenging ability of IL and the surface chemistry of MXene sheet. This study provides a new strategy to achieve chemically stable Ti₃C₂T_x and improved mechanical strength of assembled MXene film, which is also available for other type of MXenes.