Based on the earthquake instability characteristics of cohesive soil slopes, in this study, we used the stress analysis method to analyze the process of slope instability. By analyzing the stress state and shear stress variation features of soil in different parts of the slope during earthquakes, combined with the failure characteristics of actual slopes during earthquakes, we developed our proposed three-segment sliding instability mechanism of cohesive soil slopes. From our analysis results of the stress correlation between the above-mentioned mechanism combined with the strength reduction finite element method (FEM), we carried out a dynamic stability analysis of an actual loess seismic landslide. A comparison of our results with those of other methods reveals that the dynamic safety factor obtained by the strength reduction FEM, which considers the three-segment sliding instability mechanism of cohesive soil slopes, was very close to that determined by the limit equilibrium method.