In order to solve the reliability problem of structural damage equation, a damage detection method based on ridge estimation and L-curve is presented. First, a structural damage equation based on modal strain energy sensitivity is given out. Then, considering that measurement noise will cause the ill-posed equation problem, ridge estimation is utilized to solve the damage equation and L-curve is utilized to identify optimal ridge parameter. Finally, an improved ridge estimation method is proposed to enhance the identification precision. Simulation results demonstrate that the proposed ridge estimation and L-curve method can detect structural damage locations and extent with good accuracy, the quantification results of the proposed method are obviously better than those of the basic ridge estimation and L-curve method and the truncated singular value decomposition method.