Strength nonlinearity and failure modes are considered to have impact on the stability of shallow expansive soil slopes at low stresses. Based on the nonlinear characteristics of shear strength of saturated remolded expansive soil in low stress section obtained from slow shear test, the stress threshold for linear development at normal stresses and nonlinear variation at low stresses was determined by adopting the Chauvenet criterion. It was clarified that rain infiltration was the main reason that the strength of shallow sliding bodies of expansive soil slope was in the nonlinear section at low stresses. On the basis of the failure mode of “parallel slope-curve”, a stability analysis method was established to investigate the nonlinear strength characteristics of expansive soil in shallow embankment slopes at low stresses under seepage force. Results show that the mechanical properties of small cohesion and large internal friction angle of saturated expansive soil in embankment slopes at low stresses were the strength conditions of the sliding failure induced by heavy rainfall. The composite failure mode of “parallel slope-curve” based on the strength nonlinear power function model at low stresses could describe the shallow stability of expansive soil slopes more accurately, and the obtained safety factor was obviously lower than that of the circular failure mode based on the Coulomb model at normal stresses.