In order to extend the detection range of the ultrasonic guided wave testing technique and to improve the sensitivity of recognizingsmall defects, a new method of weak signal detection using Lyapunov exponents of the Duffing equation is proposed. Firstly, the mathematical principle of the Duffing equation for guided wave signal detection is analyzed, and the best way to set up the parameters of the inspection system is further researched. Based on studying the phase portraits and Lyapunov exponents of the Duffing system with and without inputting the pure noise and guided wave signal, the feasibility of the proposed method is verified. It is shown that the Duffing system is sensitive to the guided wave and has strong immunity to the noise. Thus, the guided wave signal inspection method based on the Duffing system is a valid inspection method for the weak ultrasonic guided wave. Numerical examples show that the motion states of the Duffing system can transform from strange attractor to limit cycle if the inputting guided wave signal is polluted with strong noise. The proposed method can be used to detect weak guided wave signal with strong noise. It is significant in improving the sensitivity of small defects detection using ultrasonic guided wave nondestructive detection.