Starting from the critical failure inclined section of reinforced concrete beam without stirrups, this study analyzed the force of each part on the critical failure inclined section. Through theoretical derivation and rational simplification, the value of calculation parameters was obtained, and the calculation model of shear capacity of based on mechanical balance for the reinforced concrete beam without stirrups. On the basis of the classical mechanics principle, the model has a clear physical meaning and can better reflect the influences of shear parameters including concrete strength, shear-span ratio, longitudinal reinforcement ratio and size effect, respectively. Then, the prediction accuracy and stability of the proposed shear model were evaluated based on 9 test specimens by comparing with the GB 50010—2010, ACI 318-14, EC 2, JSCE 2007 and Zsutty calculation formula. Finally, the applicability of the proposed model in the calculation of shear capacity of FRP reinforced concrete beams without stirrups was verified. The results show that the proposed model based on mechanical balance can effectively predict the shear capacity of reinforced concrete beams without stirrups and exhibit the shear failure mechanism of beam oblique section. Moreover, the proposed shear model has a higher prediction accuracy and stability, and can better reflect the nonlinear relationship between shear capacity versus shear-span ratio and longitudinal reinforcement ratio. In addition, the predicted results have a consistent stability with the change of shear parameters, so it can be applied to the shear capacity calculation of FRP reinforced concrete beams without stirrups.