Abstract: To address the low accuracy of influence lines in diagnosing section cracking in existing beam bridges, a nonlinear crack damage diagnosis method based on rotation angle influence lines is proposed. The effectiveness of deflection and rotation angle influence lines in damage identification is compared, along with the mechanical behavior of the constant-stiffness spring, of the open-crack and, of the breathing-crack models. The research results show that the rotation angle influence line has higher sensitivity, stability, and noise resistance, while the breathing crack model can better represent the real crack behavior. The nonlinear stiffness function of the breathing crack is introduced into the theoretical derivation of the rotation angle influence line. An analytical solution considering section cracking is established, and a corresponding diagnosis index is proposed. Numerical simulations and practical bridge tests are conducted to verify the validity of the analytical solution and the accuracy of the diagnosis index. The results demonstrate that the rotation angle influence line can accurately describe the effect of cracking on bridge stiffness, and the proposed diagnosis index can precisely locate section crack damage with good engineering applicability.