In this paper, a high-sensitivity Pb2+ on-line sensor based on black phosphorus (BP) integrated tapered fiber is proposed. The BP nanosheets which are deposited on the surface of the tapered fiber by light deposition technology are obtained by one-pot solvothermal reaction and ultrasonic peeling in the laboratory. Due to the electrostatic adsorption between BP and Pb2+, the effective refractive index (RI) of the tapered fiber surface is changed by the adsorption complexes, which eventually causes the interference wavelength to shift. The experimental results show that the sensor has a sensitivity of 0.03714 nm/ppb when the range of Pb2+ solution concentration is 0.1–105 ppb, and its limit of detection (LOD) is 0.0206 ppb. The response time of 1 ppb and 106 ppb Pb2+ solution is 44 s and 49 s, respectively. Meanwhile, the sensor has good stability after repeated measurement of the same concentration solution. In general, the sensor has the characteristics of simple preparation, high sensitivity, low detection limit, integration, and low-cost, and has potential application prospects to detecting trace Pb2+ in water quality monitoring.