The traditional theory of soil arch effect is developed on either cohesionless soil or saturated cohesive soil. In practice the unsaturated soils are commonly encountered, whose mechanical properties are influenced by natural environment transformation. Based on the Terzaghi’s assumption and the principle of principal stress axes rotation on soil arch effect, an analytical model is proposed to evaluate the loosening earth pressure of unsaturated soils considering matric suction. The analytical solutions of the loosening earth pressure and the lateral pressure coefficients are derived for uniform, trapezoidal, upright-triangular and reverse-triangular matric suction distributions respectively. A numerical model by FLAC is developed to verify the analytical model. The theoretical results are consistent well with the numerical results. The influences of such factors as saturation degree, thickness of overlying soil, Trapdoor width, groundwater level and rainfall, on loosening earth pressure are discussed. It is found that the loosening earth pressure decreases firstly and then increases with the saturation degree of soil increasing. The minimum loosening earth pressure is reached at the saturation degree corresponding to the air entry value of the soil.