Abstract: In order to reveal the horizontal load transfer mechanism in the composite cable tower anchorage zone with exposed steel anchor box, a theoretical calculation formula for the relative slip between steel anchor box side plate and concrete tower wall is derived upon the elastic medium layer method, and then the calculation methods of shear flow of connectors and axial forces of steel and concrete members are obtained. Through the comparison of plane section assumptions, theoretical calculation formulae and segment model test results of anchorage zone, the rationality of theoretical calculation formulae proposed is verified, and parametric influence analysis is carried out. The results show that: the maximum relative slip of the steel-concrete interface is 0.055 mm, the maximum shear force of the stud connectors is 15.99 kN, the tensile stress of the steel anchor box side plate is 6.55 MPa, and the compressive stress of the concrete tower wall is −1.07 MPa. The proportion of prestress along the bridge direction between steel anchor box side plate and concrete tower wall is 10% and 90%, and the proportion of horizontal cable force is 26% and 74%. The horizontal force transfer mechanism of anchorage zone is greatly influenced by the thickness of steel anchor box side plate, by the thickness of concrete tower wall, by the force transfer length of steel-concrete interface, by the shear stiffness of connectors and, by the prestress along the bridge direction.