In order to avoid the frequent occurrence of slope failure, it is necessary to study the causes and prevention measures of slope failure. Taking the excavation instability of a highway rock slope in Guizhou province as an example, the instability of rock slope was analyzed, and 3DEC was used to analyze the supporting effects of two reinforcement schemes. The following conclusions are drawn: (1) excavation is the main factor inducing the instability of rock slope. (2) Based on the two reinforcement schemes of 3DEC, the supporting effect is analyzed from two aspects of rock mass displacement and maximum shear strain increment, and it is concluded that the slope is in a stable state, but the supporting effect of scheme 1 is better.