Light gauge steel-concrete mixed structure is a novel structural system which not only suits the Chinese practice but also meets the industrialization and modularization of building construction. The light-gauge steel substructure of the mixed structure system will increase the lateral stiffness of the main concrete structure. When the simplified model with only the main structure is adopted in design analysis, this effect can be taken into account by the period reduction factor. The simplified formulas of both the lateral stiffness of 2 or 3 story light steel wall substructure and the period reduction factor of the mixed structure were derived in this paper. Based on the test results, the refined and the simplified model of the light steel-concrete mixed frame were established by using ABAQUS software. The numerical results agree well with the experimental ones. Finally, the proposed simplified formulas of substructure lateral stiffness and period reduction factor were verified using FEM stimulations.