Abstract: The structure of stainless steel sandwich panel is a new type of building structure system similar to the honeycomb plate. The structure is formed by brazing a thin-walled pipe arranged at a certain distance between the upper and lower panels. In the paper, the deflections at the mid-span of four-side simply supported unidirectional plates of the structure of stainless steel sandwich panel were studied by experiments, finite element analysis and theoretical analysis. Theoretical analysis was based on the principle of virtual work. Through the simplification of the mechanical model of stainless steel sandwich panel, the formula for calculating the deflection of the four-side simply supported unidirectional plate with the structure of stainless steel sandwich panel subjected to concentrated load at the mid-span was derived. Through the test results, the deflection calculation formula was verified. Finally, the influence of different parameters on the deflection of the four-side unidirectional plate of stainless steel sandwich panel was analyzed. The research results showed that the deflection calculation formula based on the virtual work principle could accurately calculate the mid-span deflection of the four-side simply supported unidirectional plate of the structure of stainless steel sandwich panel. In addition, the outer diameter of the core tube, the longitudinal spacing, and the thickness of the panel had a greater impact on the mid-span deflection.