The shallow expansive soils, closely related to the engineering disaster of expansive soils, are usually subjected to low confining pressures, and their mechanical properties are different from those under high confining pressures. Triaxial shear tests were conducted to compare the shear behavior and nonlinear characteristics of the shear strength of expansive soil under high and low cell pressures. The shear properties and reinforcement mechanism of shallow expansive soil improved by polypropylene fibers were explored. Results showed that the shear strength of shallow expansive soil exhibited significant nonlinearity, which could be represented by a power function. Moreover, it was observed that the nonlinearity of the shear strength of the expansive soil became more prominent with the increase of mass fraction of water and the decrease of cell pressure. Adopting shear strength parameters obtained from triaxial shear tests under high cell pressure led to an overestimation of the cohesion of saturated shallow expansive soil by 225.7% and an underestimation of the internal friction angle by 42.5%. The shear strength of expansive soil was significantly enhanced by adding fibers, and the degree of enhancement was related to the formation of three-dimensional fiber network within the soil. Longer fibers were more effective in reducing the attenuation rate of shear strength, which weakened the strain-softening effect of expansive soil. However, excessively long fibers may bend and twist within the soil, leading to the degradation of the fiber-expansive soil interface during soil deformation.