Laboratory physical simulation test with similar materials is an effective tool to study the practical problems in mining and civil engineering. This paper conducted an orthogonal study on low-strength similar materials comprising sand, fly ash and plaster, and analyzed the sensitivity of the materials. It has been found that the mechanical properties of the similar materials extremely depend on the proportioning ratio, and they satisfy the requirements of different laboratory physical simulation tests with similar materials. In addition, the compression strength and elastic modulus of the similar material decrease as the sand–binder ratio or cement ratio increases. The relation between the sand–binder ratio and compression strength of the material and that between the sand–binder ratio and elastic modulus are linear. Meanwhile, the relation between the cement ratio and compression strength and that between the cement ratio and elastic modulus are both exponential. Sensitivity analysis using the range method shows that the effects of the cement ratio on the compression strength and elastic modulus are more significant than the effects of the sand–binder ratio. Finally, one of these similar materials was used in a simulation test on coal backfill mining effectively.