Abstract: The three-dimensional swelling pressure tests on highly expensive soil are carried out under different dry densities, initial moisture contents and temperatures using composite modification methods. The results show that the ratio of vertical swelling pressure to horizontal swelling pressure increases with the increase of dry density in the range of test conditions (dry densities of 1.2, 1.35, 1.5 and 1.65 g/cm³, initial moisture contents of 15%, 18%, 21% and 24%, temperatures of 20℃, 35℃, 45℃, 55℃ and 65℃). The ratio is close to one when the dry density is 1.2 g/cm³. Moreover, the dry density is the key factor that determines the swelling pressure equilibrium time. The larger the dry density, the longer the equilibrium time. However, the swelling pressure equilibrium time of all samples is within 3000 minutes. The vertical swelling pressure of the expansive soil increases exponentially with the enlargement of the dry density at room temperature of 20℃ when the initial moisture contents are the same. Under the same temperatures and dry densities, the three-dimensional swelling pressures decrease linearly with the increase of the initial moisture content. Moreover, the slope of the relation curve increases linearly with the dry density. The vertical swelling pressure of the soil decreases by 33.3%, which is treated by 0.8% polyvinyl alcohol (PVA) with 5% potassium chloride (KCl) mixture solution and 0.6% polypropylene fiber, and that of the soil reduces by 16.7%, which is treated by 0.8% PVA with 4% lime (CaO) mixture solution. The vertical swelling pressure of the unmodified soil and the modified sample treated by 0.8% PVA with 4% CaO mixture solution decreases with the rise of the temperature. However, the vertical swelling pressure of the soil samples increases with the rise of temperature, and gradually decreases when the temperature exceeds 45℃ after modification by 0.8% PVA with 5% KCl mixed solution and 0.6% polypropylene fiber.