The　shallow　instability　of　granite　residual　soil　slope　could　easily　induced　by　rainfall.　Based　on　the　analyses　of　influence　of　rainfall　on　the　water　content　of　slope　soil,　the　effective　rainfall　resulting　in　soil　from　unsaturated　to　saturated　condition　is　derived　from　the　relationship　between　saturation　and　water　content.　Results　show　that　not　all　rainfall　will　cause　the　instability　of　soil　slope,　and　only　when　the　soil　saturation　caused　by　rainfall　infiltration　reaches　a　certain　degree　the　slope　fails.　There　exists　a　critical　depth　in　the　unsaturated　and　saturated　zone　of　slope　above　which　the　rainfall　infiltration　leads　to　a　transition　in　soil　from　unsaturated　to　saturated,　different　shear　strength　could　be　used　to　calculate　the　stability　of　soil　which　was　divided　into　fluctuation　two　parts　by　the　critical　depth.　The　soil　unsaturated　zone　and　saturated　zone　can　be　taken　to　calculate　the　stability　of　the　soil　condition.　The　results　are　useful　for　determining　rainfall　threshold　of　slope　instability,　so　as　to　further　understand　the　mechanism　of　landslide　instability　and　landslide　prediction.