Abstract: For low-permeability reservoirs, interfacial tension, rock wettability and pore-throat ratio are important factors affecting production and recovery. Wettability index and wetting angle have a direct quantitative relationship with oil displacement efficiency. This paper presents a concept of modified capillary number associated with interfacial tension, rock wettability and pore-throat ratio, and establishes a wetting angle characterization method of low-permeability reservoir based on Amott wettability index and USBM index. We also derive a capillary number expression for low-permeability reservoirs and determine the relationship with residual oil saturation through core test. Based on this, the calculation formulas of a relative permeability curve, oil displacement efficiency and injection ability are given for different conditions of interfacial tension and wetting angle, which reveals the relationship between reservoir physical characteristics. The study provides a theoretical basis for optimizing chemical flooding, changing interfacial tension and wettability controlling parameters in low-permeability reservoir, and further improves oil displacement efficiency and recovery.