A rainfall infiltration test model which enables the measurement of the volumetric water content of soil is designed. The distribution and variation of the initial water content under rainfall infiltration are obtained through the test. Based on these results, a novel method for shallow stability analysis of earth slopes subject to rainfall infiltration is proposed by combining the unsaturated shear strength theory and the limit equilibrium method. The proposed method is then applied to study the stability of a soil slope along the Qili connecting line of Hang-Xin-Jing Expressway. The main findings are as follows: The inverse proportion function characterized by water content distribution parameter A, correction coefficients λ and μ can be used to describe the initial water content distribution of soil in natural state. With the increase of A value, the initial stability of the slope decreases, and the sensitivity of rainfall to slope instability also decreases. When the rainfall intensity is the same, the greater the initial surface water content, the shorter the rainfall time needed to cause the slope instability, and when rainfall time is the same, the greater the initial surface water content, the smaller the rainfall intensity needed to cause the slope instability. Under the same groundwater level, the rainfall intensity is inversely proportional to the rainfall time needed to cause the slope instability.