Abstract: The water sensitivity of clay minerals is one of the primary causes of reservoir damage in oil fields. As a new testing method of expansive soil, the relative free swelling ratio is defined as the ratio of the equilibrium sediment volume of 10 g ovendried soil in distilled water to that in coal soil (or CCl4). This paper applied this new testing method in swelling property analysis of clay minerals in sandstone reservoir. The relative free swelling ratios of 27 sandstone core samples with argillaceous contents of 10~50% in Shengli Oil Field were measured, the clay minerals contents were also identified by X-Ray, and then the positive correlation model between montmorillonite contents and free swelling ratios of sandstone reservoirs was established. The results showed that the free swelling ratio measurement can more directly and accurately describe the free swelling properties of clay minerals in sandstone reservoirs, and quantitatively reflect the potential swelling damage of reservoirs caused by water sensitivity of clay minerals, and consequently can be applied in the development of clay antiswelling agents for reservoirs, which is important and significant for adjusting the development plans of oil fields.