Abstract: In order to measure the vertical swelling ratio and lateral swelling pressure under different vertical loading and confined conditions during soaking, the conventional confined swelling test apparatus and method are modified to ensure the consistency of the initial water content and the dry density of specimens before soaking and loading, getting rid of the initial horizontal stress caused by specimen preparation and distinguishing the lateral swelling pressure from the total lateral pressure. Using the modified apparatus and method, the conventional and modified confined swelling parallel tests are respectively carried out on the expansive soils with medium swelling potential from Baise County in Guangxi Province with specific water content and dry density. The test results show that the initial water content and dry density will change due to vertical loading during conventional tests, resulting in underestimating vertical swelling ratio measured by 7% to 95%, and that the existence of the initial horizontal stress produced by specimen preparation will underestimate the lateral pressure measured by 16.1% to 43.5%. The results of multi-stage loading and step unloading tests show the great consistency and repeatability. The vertical swelling ratio decreases with the increase of the vertical loads, and a fitting formula performing the ultimate swelling state based on power function is set up by relative swelling ratio and relative swelling pressure. While the lateral swelling pressure increases with the increase of the vertical loads, reaching its maximum value with the increasing vertical loads to the vertical swelling pressure, a hyperbolic function can fit well between the upper loads and the lateral swelling pressures. The achievements may provide references for establishing a swelling model and designing retaining structures in expansive soil areas.