Abstract: As theoretical research and engineering applications continue to advance, partially encased steel-concrete composite beams(PEC beams) have been shown to exhibit not only superior fire resistance but also significantly enhanced load-bearing capacity, stiffness, and resistance to buckling when compared to traditional steel-concrete composite beams. In order to study the mechanical performance and stiffness calculation methods of PEC beams subjected to hogging bending moment, static bending tests on three PEC beams with different force ratios and one steel-concrete composite beam were conducted. In addition, the effect of reinforced concrete between the flanges of steel beam on the bending stiffness under negative bending moment was also analyzed. The experimental results suggest that the flexural capacity and stiffness of PEC beams subjected to hogging bending moment compared with steel-concrete composite beam are increased by 40% and 25%, respectively. The ultimate bearing capacity of PEC beams increases with the increase of the force ratio. The width of cracks observed in the flange concrete of PEC beams is significantly smaller than that of normal composite beam. Furthermore, it has been observed that the crack width in both the flange concrete and web concrete of PEC beams decreases as the cross-sectional area of the longitudinal reinforcement increases. When subjected to negative bending moments, the section stiffness of the composite beams experiences a reduction due to the occurrence of tensile cracking within the concrete. As these cracks continue to propagate, the contribution of the web concrete to the overall stiffness of the beam gradually diminishes. Nevertheless, given that the web concrete is confined by the steel beam and that cracks have not fully penetrated it, concrete between the flanges remains capable of contributing to the stiffness of PEC beams. When the load approached its maximum capacity, the load-deflection curves gradually flatten which indicates favorable ductility characteristics of PEC beams. During the initial stages of loading, composite beams conform to the plane section assumption. However, as the load increases, slippage between the concrete slab and steel beam increases, which causes the difference in strain at their connection interface gradually increases. Considering the influence of reinforced concrete between the flanges of steel beam on the stiffness of composite beam, as well as slip effect of the interface between steel beam and concrete slab, the formulas for calculating the bending stiffness of PEC beams are proposed based on experiments. The test results verify the accuracy of the proposed calculation methods.