To solve the deformation and stress of the steel-concrete composite beam under the impact of interface slip and shear deformation, Timoshenko beam assumption is introduced to establish theoretical calculating model of simple steel-concrete composite beam in elastic state, which leads to the analytical solutions of deflection and section stress under mid-span concentrated load. By analyzing the test results of four simple composite beams with different connection degrees and comparing it with theoretical results, the value and the influence on deflection of interfacial shear stiffness are discussed. Results show that (1) theoretical results and experimental data are in good agreement； (2) the interface slip increases deflection, and the interfacial shear stiffness has an obvious influence on it； (3) neglecting shear deformation is unsafe to some extent when calculating the deflection of the composite beam.