Abstract: The double steel plate concrete composite structure (SC structure) connects the outer steel plate with the inner concrete through studs, fully utilizing the advantages of both steel and concrete materials. It has been widely used. In this study, the Planar Bi-directional Element Tester (PBET), a new type of simple thin film testing machine independently developed by Tsinghua University, was used to conduct complex in-plane stress performance tests on SC structural elements. The failure modes and bearing capacity data of the specimens under various loading conditions were obtained. The bearing capacity of the specimens was verified using both Chinese and American standards, and the failure strength envelope of the specimens was obtained. Based on the experimental results, the design envelope formulas for the bearing capacity of SC structures applicable to complex in-plane stress conditions have been proposed. Compared with Chinese and American standards, the formulas are more in line with the test values and tends to be safer.