Objective To address the intensified steel corrosion that accompanies microbial enhanced oil recovery (MEOR), a strain of Bacillus velezensis (designated 4-19) was isolated from produced water in the Changqing Oilfield. Its oil-displacement efficiency and the synergistic corrosion-inhibition mechanism on J55 carbon steel were investigated. Methods Crude-oil viscosity reduction, SARA (saturates, aromatics, resins, asphaltenes) fractionation and gas chromatography were employed to characterize the degradation of heavy fractions. Weight-loss measurements, electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM) were used to elucidate the corrosion-inhibition behavior. An equivalent circuit model combined with energy-dispersive X-ray spectroscopy (EDS) was applied to reveal the protective film mechanism. Results Strain 4-19 lowered the crude-oil surface tension by 54.5 % and achieved a 69 % degradation rate within three days. Viscosity at 35 °C and 40 °C was significantly reduced; the relative molecular masses of aromatics and resins decreased by 17.61 % and 18.60 %, respectively. The corrosion rate of J55 steel declined by 76.23 %. EIS indicated that the film resistance (Rb) increased by 42.6 % and the polarization current density decreased by 76.7 %. Conclusion B. velezensis 4-19 secretes lipopeptide biosurfactants that degrade heavy fractions while simultaneously consuming dissolved oxygen, forming a compact corrosion-product film. This coupling of oil recovery and corrosion inhibition offers a novel strategy for field-scale MEOR applications.