Abstract: The GFRP-concrete composite beam, i.e. combining the GFRP tube with concrete slab, is available to work by means of the shear connectors as flexible connectors. However, the connectors will be deformed when transferring interfacial shear force, thus causing the slip between GFRP tube and concrete slab. The interfacial slip will result in the stress redistribution between GFRP tube, concrete flange plate and shear connectors. Based on the principle of minimum potential energy and according to the structural and load bearing characteristics and variational principle, some differential equations of axial force are derived taking account of the effect of interfacial slip, as well as the theoretical formulae of the axial force acting on both the concrete slab and GFRP tube in the composite beam under symmetrically concentrated load. The calculated results showed that the axial force acting on the composite beam increases with the increasing connector stiffness and external load and decreases with the increasing wall thickness of GFRP tube. And the maximum axial force is found at the midspan, then it decreases gradually from midspan to beam end where it is approximately zero.