Abstract: Soft soil foundation is characterized mainly for low bearing capacity, large deformation, especially non-uniform deformation. The deformation stability can last for a few years or even decades. After the bridge foundation built in soft soil is completed, the place such as bridge foundation, subgrade, et al.appear significant differential settlement, and cause structural crack damage and tilt. Therefore, one overpass bridge foundation for soft soil was chosen in southwest China as the research object. The detailed geological investigation was conducted in study area. Physical and mechanical properties of the soil and the mineral and chemical composition in soft soil areas were measured and analyzed to obtain the soft foundation engineering characteristics and provide valuable reference for practical engineering. The conclusions and suggestions are expected to be applied to other road construction in soft soil district in southwest China.