It is critical for fabricating flexible biosensors with both high sensitivity and good selectivity to realize real-time monitoring superoxide anion (O₂^•−), a specific reactive oxygen species that plays critical roles in various biological processes. This work delicately designs a Mn₃(PO₄)₂/MXene heterostructured biomimetic enzyme by assembling two-dimensional (2-D) Mn₃(PO₄)₂ nanosheets with biomimetic activity and 2-D MXene nanosheets with high conductivity and abundant functional groups. The 2-D nature of the two components with strong interfacial interaction synergistically enables the heterostructure an excellent flexibility with retained 100% of the response when to reach a bending angle up to 180°, and 96% of the response after 100 bending/relaxing cycles. It is found that the surface charge state of the heterostructure promotes the adsorption of O₂^•−, while the high-energy active site improves electrochemical oxidation of O₂^•−. The Mn₃(PO₄)₂/MXene as a sensing platform towards O₂^•− achieves a high sensitivity of 64.93 μA·μM⁻¹·cm⁻², a wide detection range of 5.75 nM to 25.93 μM, and a low detection limit of 1.63 nM. Finally, the flexible heterostructured sensing platform realizes real-time monitoring of O₂^•− in live cell assays, offering a promising flexible biosensor towards exploring various biological processes.