In order to investigate the influence of strength reduction of loess induced by rainfall infiltration on the stability of tunnel, direct shear- and triaxial- tests were carried out on loess.The results show that the water content has significant influence on the mechanical behaviors of loess.The elastic-plastic incremental constitutive relation of undisturbed soil was established.By compared with experimental data, the constitutive model was proved to be capable of describing the stress-strain relationship of loess.The distribution characteristic of water content in different depth land under different rain intensity were analyzed.Besides, the constitutive model for loess with different water contents were established so as to calculate the stress and deformation of structure in tunnel under different rain intensities.The results show that when considering rainfall infiltration, the deformation of tunnel and the stress of preliminary support structure are significantly increased.In this situation, the settlement of the tunnel vault increases 46% and the stress of preliminary support structure increases 20%~27%.It is found the elastic-plastic incremental constitutive relation is reasonable and results of the numerical simulation is reliable with comparing the calculated and measured data of tunnel deformation.