The difference of strain distribution, interface slip, stiffness, ultimate strength and other mechanical properties between a new type of external cable steel-concrete composite beam and ordinary steel-concrete composite beam is studied. The test results show that the rigidity and ultimate strength of the steel box-concrete composite beam with external cable increase by 54.15% and 27.72% compared with the ordinary steel box-concrete composite beam. Due to the effect of external cable, the brittle damage degree of the steel box-concrete composite beam is reduced, the strength and stiffness of the structure are increased, and the strain distribution and growth of the steel box-concrete composite beam are more reasonable. Therefore, the composite beam with external cable steel box and concrete has better mechanical properties. The experimental results show that the strain distribution of the section is nonlinear along the width direction, and the shear lag effect changes with the increase of the load, but is not constant. It is also found that the external cable stress is closely related to the maximum compressive strain of the concrete wing slab. On the basis of the experimental research, the section nonlinear analysis model and the calculation formula of the ultimate strength of the external cable-steel box-concrete composite beam are established. The calculated ultimate strength is in good agreement with the experimental results, which provides a meaningful reference and research way for the theoretical analysis and practical application of the ultimate strength of the external cable-steel box-concrete composite beam.