The shear failure mode and failure mechanism of 12 CFRP-strengthened concrete beams without stirrups were analyzed. The stress-strain curves of the CFRP beams in the whole process were obtained. The quantitative effects of structural size and CFRP ratio on the shear strength of CFRP are discussed. Furthermore, a formula for calculating the shear strength of CFRP is proposed and compared with the test results, the test results of others and the standard results in this work. It is found that the structural size has a significant influence on the shear strength of CFRP. The nominal shear strength of CFRP decreases significantly when the structural size increases. In addition, when the beam depth changes from 300 mm to 1 200 mm, the nominal shear strength of CFRP decreases by about 50%, which has an obvious size effect. Furthermore, under different cross-section sizes, the nominal shear strength of CFRP increases with the increase of CFRP ratio ρf , but with the increase of CFRP ratio ρf, the increase amplitude of nominal shear strength decreases, showing a nonlinear growth trend. Finally, the established formula for calculating the shear capacity of CFRP could predict the shear capacity of CFRP under different CFRP ratio and section size scientifically.