In order to study the plate width-thickness ratio limits of steel palte based on section classification for Steel-Concrete Composite Beam with U-section (SCBU), monotonic static loading tests were conducted on five SCBU specimens. The damage modes, load-displacement curves and plastic deformation capacities of SCBU with different combinations of height-thickness ratios of outer steel-encased web in compression area and width-thickness ratios of compressed flange were analyzed. The test results show that the width-thickness ratio of the steel plate has a direct influence on the buckling deformation and plastic deformation capacity of the SCBU. All the specimens suffered local buckling before reaching the peak bearing capacity, the failure mode was bending plastic damage. Meanwhile, the finite element analysis results show that the nonlinear behavior between the steel plate web and compressed flange interacts with each other, the buckling moment of the plate advances with the increase of width-thickness ratio of the adjacent plate, and the degree of plastic development of the SCBU decreases with the increase of width-thickness ratio of steel plate. Based on the results of the test and finite element parametric analysis, the recommended limit value of width-thickness ratio for class II section (Class 2) steel palte of SCBU is proposed and compared with the code limit values.