Abstract: Primary octadecylamine was used to repellent the remolded expansive soil, and the moisture content was 5%, 10%, 15%, 20% and modified expansive soil with octadecylamine content of 0%, 0.2%, 0.5% and 0.8% respectively. The water repellency grades of the modified expansive soils were measured by the drop water penetration time method, and the corresponding unloaded and charged swelling rate tests were carried out. The effects of octadecylamine content and moisture content on the swelling deformation of the expansive soil were obtained. The results show that the modified expansive soil with an octadecylamine mass fraction of 0.8% has a drip penetration time exceeding 3600 s, which owned an extremely water-repellent grade. When the octadecylamine content increased from 0% to 0.8%, the stability duration of no-load expansion was shortened from 97 h to 47 h, and the expansion rate decreased from 12.9% to 9.4%. When the octadecylamine content is 0.8% and the moisture content of modified expansive soil is 20%, its charge swelling rate decreases from 2.14% at 50 kPa to 0.01% at 400 kPa. When the load is small, the octadecylamine content and the initial moisture content have a greater influence on the swelling rate of the load. When the load is large, the load is an important factor affecting the expansibility of expansive soil.