An experimental study has been performed on Steel-Concrete Composite Castellated Beam (SCCCB). The basic mechanical performances of SCCCB, such as stress and deformation, were the focus of this study. The test results have indicated that, due to the influence of secondary bending, the peak stress is achieved in porthole. Based on vierendeel truss theory, the formulas considering the effect of ribbed stiffener to calculate the stress of SCCCB were proposed, the theoretical results are consistent with the test results, and the results indicate that the maximum stress of perforated web appears at the angle of 150~165°. Also, a formula for the evaluation of the deflection of SCCCB was derived according to vierendeel truss theory. It is found that this method can satisfy engineering requirements by comparing theoretical results with both experimental and FEM ones. The analytical method can be used for the serviceability limit state design of SCCCB.