Ultrathin 2D Bi3O4Cl nanosheets are promising photocatalysts for photocatalytic organic pollutions degradation, and tailoring the electronic structure by non-metal element doping of Bi3O4Cl is an important strategy to increase its photocatalytic activity. However, the role of doped non-metal atoms on charge carriers separation and light absorption has not been understood in depth. Here, the B-doped Bi3O4Cl ultrathin nanosheets are fabricated via a solvothermal way, which increase solar absorption and electron-hole separation of Bi3O4Cl. The products are characterized by FE-SEM, TEM, AFM, indicating that B-doped Bi3O4Cl are 3.87 nm thick nanosheets. And UV–Vis-DRS, XPS, PL and density functional theory show that the doped B atoms play multiple roles in facilitating photocatalytic performance: (1) inducing midgap states to immensely expand the light response region up from 450 nm to 557 nm; (2) acting as the electron capture centers to accelerate charge carries separation. The ESR technology shows that B-doped Bi3O4Cl can produce more O2– and OH radicals. As a result, the B-doped sample achieves a high-efficient photocatalytic bisphenol A and ciprofloxacin degradation, 3-fold and 2.1-fold higher than pure Bi3O4Cl, respectively. This work presents new opinions into the design of photocatalyst and confirms the role of electronic structure modulation on tuning catalytic activity.