Aiming at the non-contact detection needs of nonlinear effects of metal fatigue damage and its low signal-to-noise ratio, an electromagnetic ultrasonic scheme is proposed to collect signals for nonlinear effect. The Duffing chaotic system is used to quantitatively assess the fatigue degree of metal. Then, the nonlinear characteristics of material fatigue evolution are characterized according to the phase trajectory diagram and the Lyapunov index. In this article, the evolution process of fatigue damage of aluminum alloy is analyzed by the finite element method, and the relative nonlinear coefficient change law during the evolution of fatigue damage is studied based on the material Murnaghan model and the micro-crack equivalent spring model. In addition, the noise immunity of the Duffing chaotic system for the extraction of nonlinear effect features is investigated. When the signal-to-noise ratio is 20 dB, the relative nonlinearity coefficient error is 132. 12% , while the Lyapunov index error is 8. 82% . Therefore, the Lyapunov index has significant noise immunity compared with the nonlinear coefficient. In addition, based on the experimental study of fatigue detection of aluminum alloy, the feasibility and accuracy of the Lyapunov index characterization and analysis method of nonlinear effect of electromagnetic ultrasound are evaluated. Results show that the Lyapunov index can effectively address the problem of low signal-to-noise ratio in the process of electromagnetic ultrasonic nonlinear detection. In this way, the sensitivity and repeatability of nonlinear feature picking are improved, and the contribution of non-contact ultrasonic detection methods is further enhanced, such as electromagnetic ultrasonic in the evolution of fatigue online detection.