Composite bolts containing steel bars have been successfully applied for the anchorage of earthen sites. However, the mechanism of anchoring has not been studied yet. In the Jiaohe ruins (a typical earthen site), composite bolts containing 20mm diameter steel bars were chosen for pullout experiments and to monitor the interface strain along the bolt. The experimental results indicate:1) the maximum anchoring force of a 3m long bolt is over 150kN, and the bolt shows strong plastic deformation; 2) at the damage interface, the axial strain of the interface between steel and composite material is greater than at others; 3) due to the heterogeneity of the bolt, the irregularity of the axial strain occurs at the interface between bamboo and composite materials, which creates some local compression phenomena; 4) the change of shear strain along the bamboogrout interface and the bamboocomposite material interface coincides with that of the pullout load, and shear strain is transferred to the end of the bolt under higher loads; 5) because of the multilayer structure of the bolt, transverse transfer and shearing expansion appear distinctly during the pullout experiment.