Dispersive soil has the characteristic of being dispersed and lost when encountering water. In engineering practice, lime and cement are often used to modify dispersive soil, but doing so often has such problems as environmental pollution. The Si/Ca compound system composed of nano silica sol and calcium chloride is selected as the modified material for dispersive soil. The effect of the modification is studied by pinhole test, mud ball test, and dispersion and disintegration test. The mechanism of dispersive soil modified by Si/Ca compound system is explored by physical and chemical properties tests. The results show that the dispersibility of soil can be completely eliminated by using nano silica sol alone at 25% dosage or using calcium chloride alone at 0.40% dosage. However, the Si/Ca compound system composed of 1% nano silica sol and 0.05% calcium chloride can also completely eliminate the dispersion of soil, which effectively reduces the dosage of the two materials used alone. The disintegration process of the soil modified by nano silica sol modified is different from that of dispersive soil, with a shorter final disintegration time and a more stable disintegration rate. The final disintegration time is further shortened and the disintegration rate increases when the nano silica sol and calcium chloride are used together. The mechanism of dispersive soil modified by Si/Ca compound system includes reducing the percentage of exchangeable sodium ions and the pH of the soil, and generating calcium silicate hydrate. The results demonstrate that the Si/Ca compound system composed of nano silica sol and calcium chloride can effectively modify the dispersive soil.