Abstract: To improve the crack resistance of ordinary concrete and address issues such as excessive deformation and wide diagonal cracks in fiber-reinforced polymer (FRP) reinforced concrete beams, steel fiber-reinforced concrete was used to prepare glass fiber-reinforced polymer bar-reinforced steel fiber-reinforced concrete (GFRP–R–SFRC) beam specimens. Shear tests were conducted on the GFRP–R–SFRC beam specimens to investigate the influence of steel fiber content and shear span ratio on the distribution and width of diagonal cracks. On this basis, considering the characteristics of GFRP stirrups and the influence of steel fiber content, the calculation method for GFRP stirrup stress was modified, and a formula for predicting the maximum width of diagonal cracks in the specimens under shear was proposed. The results show that incorporating steel fibers can significantly inhibit the initiation and propagation of diagonal cracks in GFRP–R–SFRC beams, although a larger shear span ratio is not conducive to the effectiveness of steel fibers in crack suppression. Within a certain range of fiber content, under identical shear conditions, the width of diagonal crack in GFRP–R–SFRC beams decreases with increasing fiber content. Compared to specimens without fiber, the maximum diagonal crack width of beams with 1.5% (volume fraction) steel fiber content is reduced by 24.3%. The calculated results from the proposed maximum diagonal crack width formula agree well with the experimental results.This study provides an important reference for the practical application and promotion of GFRP–R–SFRC beams in engineering.