Abstract: Loess fill slope is prone to landslide under the condition of rainfall infiltration. The influence of compaction degree under rainfall infiltration on the deformation and failure mechanism and the instability mode and sliding mechanism of loess fill slope is explored. Based on the indoor rainfall system, combined with the sensor monitoring and 3D laser scanning technology, the rainfall model tests with slope compaction of 80% (low pressure compaction), 90% (medium compaction) and 95% (high pressure compaction) are carried out. The influence of compaction degree on the volume moisture content, matrix suction and deformation failure process of fill slope is analyzed. The results show that the first failure position of the slope is different with different compactness. The first failure of the slope under the medium and high compactness occurs at the foot of the slope, showing the collapse failure. However, the low degree of compaction is at the top of the slope, which is collapsible. The greater the compactness of the slope, the longer the duration of the deformation and failure process, and the more accumulated rainfall required, but the smaller the sliding distance and sliding surface depth. With the increase of compactness, the failure mode of the slope changes from deep overall failure to shallow multi-stage failure. The low-pressure compaction slope is of the collapsible settlement deep creep tensile failure, the medium compaction slope is of the deep creep tensile failure, and the high-pressure compaction slope is of the shallow multi-stage backward failure.