In order to investigate the formation mechanism of shear band when slope was damaged by load, an indoor model test was designed. By using the digital image correlation (DIC) method, the formation of shear bands and the progressive failure of a sandy slope were characterized and analyzed in the model test, in which failure was deliberately triggered by loading. In this paper, the discrete element software PFC3D was used to simulate a model test to further supplement and verify the DIC calculation results. The results suggest that developing a shear band is a dynamic process. At the beginning of the loading, strain localization regions were scattered around the loads. As loading continued, some of the strain localization regions connected and formed multiple shear bands, while some strain localization regions disappeared. Around the time 3/4T (i.e., T was the time when the slope reached ultimate capacity and failed), multiple shear bands transitioned into one major shear band that extended to the slope surface around time T. After time T, the horizontal displacement of the sliding body increased rapidly, and the damage phenomenon manifested as an overall downslide.