At present, there is a lack of suitable preparation method for similar materials of surrounding rock in model test of super large-span flat underground caverns. The physical and mechanical parameters for surrounding rock of class Ⅲ in engineering are taken as the target for proportioning test with similar materials. Barite powder and quartz sand were used as aggregate, high-strength gypsum powder and water were used as cementing agent, and laundry detergent was used as regulator. The experimental scheme was designed based on the orthogonal test theory. The scheme took three different factors with five different levels of each factor in account. It was made up of 25 sets of samples with different proportions. The cohesive force and internal friction angle of samples were measured by a large number of direct shear tests. The compressive strength and elastic modulus were measured by uniaxial compression test. Moreover, the sensitive factors were analyzed by range analysis method. The test results show that the prepared similar materials have a wide range of physical and mechanics parameters, which can meet the requirement for strength of most rocky similar material of model test. The change of material content has obvious influences on physical and mechanical parameters, which can provide guidance for proportioning of similar materials of different model tests.