Injection water is the main way to supply formation energy in the low permeability reservoirs，and water quality is the key influencing factor of water flooding effect. Related salinity index of the injection water was not proposed by the recommended specification of the injection water quality using in the low permeability reservoir. There are no standards for dividing the reservoir with permeability less than 10×10-3 μm2. The throat distribution difference and the contribution of the dominant throat to the permeability were analyzed through constant velocity mercury injection experiment，and the key throat intervals influencing injection water effect in different permeability reservoirs were obtained. The effects of clay particle migration and hydration expansion on seepage capacity were analyzed quantitatively. The limits of water salinity，particle size and particle concentration with different permeabilities in the low permeability reservoir were initially proposed by laboratory core physical simulation of water flood combined with throat distribution. The results show that if core permeability is lower，the injection water salinity is closer to the formation water salinity；the damaged degree on reservoir seepage capacity is deeper with larger particle size or higher water particle concentration.