In order to improve the detection accuracy of nonlinear Lamb for structural micro-cracks under background noise interference, the Duffing system and Lyapunov exponent are used to enhance and quantify the nonlinear Lamb characteristics under noise interference. Firstly, the Poincare diagram is used to determine the dynamic parameters of the Duffing system, and then the Lamb waves nonlinearly extended are input into the adjusted Duffing system. Lastly, the method uses the phase space reconstruction of the system output time series to obtain the corresponding Lyapunov exponent. Simulation results of several model data show that there is a good linear relationship between the Lyapunov exponent and the crack size even in the case of noise. This method has an obvious advantage for the identification of microcracks defects under noise interference, which is very important to improve the detection sensitivity of nonlinear Lamb wave.