As a newly developed 2-dimensional (2D) materials, black phosphorus (BP) and few-layered BP nanosheets (BPNS) have attracted significant interest due to their unique physicochemical properties. However, their electrochemical properties have been rarely studied as limited by the poor stability upon exposure to air. Here we report a new strategy to synthesize BPNS with thionine (TH) as a stabilizer via the π−π stacking interaction. The resulting BPNS/TH composite is stable over 1 week. More importantly, this composite exhibits the intrinsic peroxidase-like property, which can electrocatalyze the reduction of hydrogen peroxide (H2O2), and a H2O2 sensor with high sensitivity and broad linear range is achieved. To the best of our knowledge, this is the first report of peroxidase mimetic property of BPNS. We envision that this study can open new avenues for the broad applications of BPNS in the electrochemical fields.