Core samples of sandstone with extra-low permeability from Huimin area in Shengli oil field were selected, and the shape and composition of clay mineral in core samples were analyzed by SEM and XRD. Electrokinetic potential of clay minerals in four experimental fluids was measured, under reference to formation water’s salinity and different pH by means of Zeta Probe (potential measuring apparatus). The results show that the positive ion concentration in these experimental fluids is higher than the critical value to balance the negative ions on the face of clay minerals. That makes the positive ion in the state of over-saturated and keeps the value of electrokinetic potential positive in a wide range of pH value. We have evaluated the salt sensitivity extent of the samples, and analysed the data with electrokinetic potential of clay minerals in experimental fluids. The results show that the salt sensitivity mechanism of extra-low permeability sandstone reservoirs, with kaolinite-chlorite-illite and chlorite-illite as the main clay mineral composition, is that with the formation salinity decreasing, the thickness of water film increases, which brings about decreasing of effective pore-throat and enhancement of permeative resistance.