Abstract: The influence law of local expansion of expansive loess on the safety of tunnel support is key to keeping surrounding rock stable. Based on a loess tunnel, the temperature stress field is built to simulate the humidity stress field generated by the humidification of expansive loess, and thermalmechanical coupling module of FLAC 3D software is used to study the influence of expansive loess on the safety of support under the condition of local expansion (tunnel arch, arch waist, side wall, and inverted arch) when encountering water. The results show that: (1) Structure deformation in local expansion area is obvious; inverted arch deforms obviously not only when it is located in local expansion area, but also when affected by the expansion of other parts, thus it is the main deformation position; under the effect of local expansion, the deformation of arch crown is the largest, the deformation of the arch waist and the side wall is the second, and the wall foundation is the weakest. (2)The maximum principal stress of support in waterimmersed expansive area increases with the expansive force, and most of them increase linearly; when the expansive force is between 200 kPa and 300 kPa, the tensile stress of support structure in expansive area exceeds the standard value of C25 concrete axis tensile strength（1.27 MPa）. (3)The failure of surrounding rock in local expansion area is mainly tensile failure; the surrounding rock under inverted arch shows tensile failure under its own expansion and local expansion of other parts, which is the stress concentration part. (4) By using the method of structural safety factor, the maximum expansion pressure and deformation value of different parts of the structure are determined; the maximum local expansion pressure of the tunnel arch, arch waist, side wall， and invert arch are 223 kPa, 260 kPa, 292 kPa， and 238 kPa respectively, and the corresponding deformation is 87 mm, 36 mm, 47 mm， and 27 mm respectively.

Key words: loess tunnel, tunnel support safety, expansive loess, local expansion, supporting structure, surrounding rock failure