I-core sandwich structure has great potential in the application of hull structure construction due to its high specific strength and relatively simple manufacturing process. The topic on the study of mechanical properties of I-core sandwich structure under bending loads is of interest to structural designers since the structure is often subjected to bending loads in engineering applications. In this paper, a metallic I-core sandwich beam with longitudinal girder was designed and manufactured using laser welding technique, and finally tested under four-point bend loading. The elastic-plastic behaviors and the ultimate load carrying capacity of this novel beam structure were obtained. A numerical model was developed to investigate the mechanical properties of this novel beam structure by finite element method. The results of the numerical model were compared with experimental data. Stress components of the front face and back face in the failure process were analyzed and discussed to investigate the failure of them. Results showed that the huge local bending stresses of plate caused the failure of the front face and back face. Finally, an improved scheme for the test was proposed to provide a pure bending load, which was proved by finite element simulation. All the findings aim to guide the engineering application of this structure.