Pseudo-static method has some limitations in obtaining the minimum safety factor to evaluate the stability of slope. In view of this, we developed a method for calculating the time-history of safety factor using FISH language through the secondary development platform of the finite-difference software FLAC^3D. Via the modified Mohr-Coulomb constitutive model we simulated the slope stability of geogrid-reinforced high earth-rock dams, and analyzed the slope stability of geogrid-reinforced high earth-rock dams according to the time-history of safety factors. According to the simulation, we obtained the changes of safety factor in the presence of varying geogrid parameters (including material strength, geogrid length, and geogrid spacing), and determined the optimum layout of geogrids. Results demonstrated that we could analyze the time-history of safety factor by the time-history graph of dynamic safety factor, and also analyze the start time of the failure of dam slope. Moreover, the reinforcement effect reaches the optimum when the elastic modulus of geogrid is 2.5 GPa and the spacing of geogrids is 4 m. The simulation results of Shuangjiangkou Rockfill Dam are basically consistent with the results in the feasibility research results. The geogrid layout is proved rational, and the simulation method in the present paper is effective.