By adjusting the composition of injection water, including ionic type and strength, the goal of enhance oil recovery rate can be achieved.However, there is no clear understanding of which factor, ionic type or strength, has a greater impact.In this paper, based on the formation water of the offshore G oilfield, various brines with different ionic strengths and types were preparedBy measuring the zeta potential of rock powder and oil droplets, three-phase contact angle, interfacial tension between oil and water, emulsification properties of brines, and film strength of oil-water interface, the variation rules of the core wettability and emulsification performance of brine were systematically analyzed when the ionic strength and ionic type were changed.The exptl. results showed that ionic strength has a greater impact than that of ionic type.For the oil-water-solid (rock) system of the G oilfield, an ionic strength of 82 mM/kg is the threshold for low salinity effect.Reducing ionic strength resulted in an increase in the hydrophilicity of rock and the ability of water flooding to emulsify oil droplets, which contribute to the improvement of oil displacement efficiency.Theor., water flooding to enhance oil recovery can be achieved by only reducing the ionic strength.This understanding was verified by dual polarization interferometry (DPI) experiments and low-field NMR experiments, which showed that reducing the salinity of the injection water from 42,000 mg/L (ionic strength of 820 mM/kg) to 4200 mg/L (ionic strength of 82 mM/kg) enhanced the oil recovery by 7.9%.