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Based on a 2D discrete element method (PFC2D code) and limit equilibrium method, a new method is proposed to obtain critical slip surface and to determine the factor of safety of the slope model. After the calibration process, the excavation process on the rectangular assembly of particles can make the slope model of the DEM. As the trial acceleration of gravity increases, the critical slip surface tends to change from shallow to deep, which indicates more serious failures in the slope. With higher trial accelerations of gravity, the factor of safety calculated by this method continually increases as well. Therefore, the critical trial acceleration of gravity that places the slope in a critical state of failure is essential to ensure the results are correct. The gravity increase method is applied to render the critical state of failure for slope model. As soon as the continuous failure surface forms, the influence parameters of particles on the critical slip surface also can be determined. Connecting the centre of these particles forms the critical slip surface, which is more consistent with the actual situation in nature due to the absence of assumptions of the critical slip surface in this method. Then, the factor of safety is calculated by the Spencer method that has the ability of dealing with noncircular, complex slip surface. This method proves to be a promising tool to analyse slope stability with the DEM. The character of progressive failure process of slope can be well modelled by DEM by investigating the development of failure in the slope body. By studying temporal and spatial risk of slope instability by the total failure process, it is possible for managers and engineers to detect temporal and spatial risks so robust suggestions and mitigation measures can be made.