Debris flows of Shishu gully are induced by rainstorm, and it is important to study the impacts of moisture content and compaction rate of source-region soil on the formation of debris flow at early stage. During rainfall, source-region soil is gradually saturated and fine grain moves, which leads to the changes of soil mechanical properties such as density and strength. In this paper, we carry out direct shear test on source-region soil of debris flow in Shishu gully by using large-scale apparatus to explore strength behavior in the presence of different degrees of saturation and dry densities. We obtain the relationships of saturation and dry density vs. shear strength index c (cohesion) and φ (inner friction angle), as well as relationships of saturation and dry density vs. strength envelope, shear stress, and horizontal displacement, respectively. The results show that: 1) relationship between c and saturation shows tendency of inverse proportion, whereas c is in proportion with dry density , especially at early stage of saturation; 2) shear stress vs. horizontal displacement curve’s peak becomes obvious as the degree of saturation increases; 3) in the presence of adequate rainfall at early stage, source-region soil gradually becomes saturated and sudden drop of shear strength occurs; then, loose deposits at the surface layer are easy to be taken away by water flow, which causes the formation of debris flow. Through the experiment, we obtain the variation behavior of strength index for source-region soil of such grain size distribution.The results offer reference for the research of formation mechanism and the forecast of such debris flow.