The fractal model in association with pore airflow analysis is employed to forecast the process of one-dimension unsaturated soil seepage under rainfall. First, curves of water-soil characteristic and hydraulic conductivity coefficient of saturated soil with different fractal dimensions and air-entry values were predicted. Furthermore, according to the fractal model for unsaturated shear strength, a new formula of the critical depth for rainfall-induced failure of unsaturated soil slope is derived. Factors controlling the instability of unsaturated soil slope under rainfall are analyzed. Results show that the fractal dimension and air-entry value determine the water-physical property of unsaturated soil. The rate and depth of rainfall infiltration are affected by the fractal dimension, while the soil suction curve is influenced by the air-entry value. The coefficient of saturated hydraulic conductivity and the rainfall intensity has large impact on the instability of unsaturated soil slope. When rainfall intensity exceeds the coefficient of saturated hydraulic conductivity, pore air pressure will be generated subject to infiltration, and hence affecting the soil suction.