In order to establish a more convenient, efficient and intuitive finite element model of CFRP plate reinforced steel beam, a three-dimensional model of reinforced steel beam of carbon fiber plate is established based on the "solid-plate-shell" adhesion-slip interface model. The characteristic load, failure mode and load-deflection curve of the unreinforced steel beam and the steel beam reinforced with CFRP plate are simulated. The stress change and interface debonding process of the CFRP plate under different loads are analyzed. The influence of such factors as the shear span ratio, span-height-ratio of the beam and the elongation ratio of the CFRP plate on the interface debonding and debonding resistance of the reinforced beam is studied. The results show that:1) the length of the pure bending segment has no effect on the interfacial stress and has little effect on the debonding bending moment;2) Under the condition that the elongation ratio of the CFRP plate remains unchanged, the bending bearing capacity of the section gradually increases with the increase of the shear span ratio;3) Under the condition that the insertion length of the CFRP plate remains unchanged, the debonding bending moment first increases and then decreases with the increase of the shear span ratio. The results can provide a certain reference for simulating the debonding process of steel beams reinforced with CFRP plates and optimizing the reinforcement efficiency.