In order to study the expansive soft rock properties of Li Jia Ping tunnel in San Xi highway, the impact of water content and dry density on expansive soil three-axis strength were investigated by three-axis strength test and expansion experiments. The test results showed that water content and dry density have a significant impact on the expansive soil strength, especially on the cohesion. However, the effects of dry density on the internal friction angle is not obvious when the dry density of the expansive soil is large. The expansive soil with small initial moisture content will produce very large expansion rate and expansive force when absorbing water and becoming expanded. Therefore, anti-seepage measures should be taken to the expansive soil area. In addition, according to the idea of convergence constraint method, the characteristic curve of surrounding rock of expansive soil is deduced. The structure shows that the change of water content will cause great change of surrounding rock pressure. Based on this, further analysis of the cause of roof collapse was analyzed. Constructing during the rainy season increased the water content of expansive soil, the poor support can not resist the enormous expansive force. As a result, the roof collapsed.