The mechanical properties of soft soils were investigated through triaxial lateral unloading tests under different initial excess pore water pressures and confining pressures. The lateral unloading stress–strain, strength parameters, and initial unloading moduli of soft soils are presented and discussed in this study. The results show that soft soils abruptly undergo unloading failure. The lower confining pressure and the higher initial excess pore water pressure can result in intensive unloading failure behavior. The unloading strength of soft soils linearly decreases as the initial excess pore water pressure increases. The stress–strain curve is characterized as strain hardening. The cohesion and initial unloading moduli of soft soils drastically reduce when the initial excess pore water pressure exceeds 20 kPa. The conclusions obtained in this study can provide a theoretical reference for the design and numerical analysis of underground spaces in soft soils.