The model test study is one of the important means to explore the mechanical response mechanism of soil and structure system. Aimed at model simulation of Xi'an Metro Line 2 tunnel, this paper firstly discusses the similar scale relation of tunnel structure model test, analyzes the selected principles and the key problems of similarity material in model test. Then based on two different geometry similarity levels, the effects of geometric similarity levels on the results of stress and bending moment between the model and prototype tunnel structure are compared and discussed, and the stress error of tunnel prototype structure is more than 12%; while the error of bending moment is more than 140%. The study results show that the geometric similarity level determines the rationality design of model similarity scale, thus it becomes one of the key restriction factors whether the model test can reasonably and accurately simulate the mechanical response of prototype structure.