Abstract: Through a 3D meso-scale numerical simulation method, the shear failure mechanical analysis model for large-sized GFRP reinforced concrete (GFRP-RC) beams was established. The influences of parameters of major importance on the crack angle and on P-Δ curve of GFRP-RC beams was discussed through this model, such as shear span ratio, longitudinal reinforcement ratio, and beam depth. Based on the modified compression field theory (MCFT), a calculation model for the average crack width was established, considering the variation of crack width along the beam depth, which improves the calculation accuracy of the simplified formula for MCFT. A design method was proposed for shear bearing capacity of GFRP-RC beams without stirrups considering the influences of the shear span ratio and size effect on the crack angle and on shear capacity. The model proposed is validated by comparing with the corresponding experimental measurements of 213 GFRP-RC beams, and with the consideration of the average crack width and size effects, utilizing the model proposed will garner the satisfactory results with remarkable higher accuracy compared to current used design codes. Therefore, the improved MCFT model proposed provides a theoretical reference for the shear design of GFRP-RC beams.