The material parameters of glass fiber reinforced plastic (GFRP) bolt were defined by writing VUMAT subroutine. Orthotropic anisotropic modeling was carried out considering the non-uniformity and damage characteristics of the interface between GFRP bolt and mortar. The bond slip characteristics of the interface between GFRP bolt and mortar were simulated sectionally by ABAQUS. The distribution of axial force and interfacial shear stress of GFRP bolt were explored, and the interface mechanical properties of GFRP bolt-mortar with different diameters were analyzed. The results show that the piecewise finite element model can better reflect the bonding characteristics of GFRP bolt-mortar. With the increase of the applied load, the axial force of the GFRP bolt gradually increases, the load transfer depth gradually deepens. The anchoring effect gradually plays from top to bottom. The maximum pull force and interfacial failure displacement of GFRP bolt increase with the decrease of diameter. The critical diameter of slip failure and strength failure is 28 mm. The slip failure occurs when the diameter is larger than 28 mm and the strength failure occurs when the diameter is less than 28 mm. The anchorage coefficient K₁ of 28 mm diameter GFRP bolt is determined to be 0.155.