In the preparation of structural surface model, cement mortar is poured into the model, and then structural surface specimen is placed directly on the flowing cement mortar, resulting in uneven settlement of the specimen. However, the structural surface model must be horizontal in direct shear test; otherwise the structural surface is not perpendicular to the vertical force of shear apparatus, which means that normal stress is not perpendicular to shear stress, leading to big error or even completely wrong result compared with the actual situation. In view of this, an instrument of preventing structural surface model from settlement in direct shear test of rock mass is proposed. The instrument consists of an outer frame, active pole, clamping plate, hook, wing nuts and so on. In preparing structural surface model, the specimen is clamped tight and fixed to secure it does not move, which avoids the uneven settlement and excessive subsidence.