Based on the existing experimental results, the dynamic response of RC beams under impact loading is divided into local response stage and overall response stage. As RC beams are vulnerable to shear failure during the local response stage, this study investigates the transient response of the components in this stage. Thus, the effect of inertial force and reverse reaction on the internal force distribution at the local response stage is also discussed. Based on the basic principles of force equilibrium and parametric analysis results, a computing method for the internal force of RC beams is proposed. The results show that both the inertial force and negative reverse reaction affect the internal force distribution of beams during the local response stage, and the reverse reaction is approximately linear to the span-depth ratio and peak impact force. The assumption that the distribution of inertial force is linear can well reflect the actual force state of the beam.