An evolution model considering rheologic effects is proposed to describe the progressive failure process of soil slopes. The proposed method is illustrated using previously published examples and analyses. Some of the results obtained are impossible in conventional methods of stability analyses. To obtain a visual understanding of the mechanical mechanism of progressive failure and the evolutionary process of the local failure of slopes, a numerical simulation is presented for the whole failure process. In this way, the whole evolution process is clearly shown, from slopes of local failures near the toe, through the appearance of tensional cracks at the top, to the final landslide. This understanding of the failure mechanism is very helpful for the prediction and prevention of landslides.