Abstract: This paper studies the weak signal detection by using Duffing oscillator. It is not only of the theoretical value but also of the engineering value to decide the existence of the weak signal in strong noise environment quickly and exactly. Considering that most of the existing weak signal detection methods based on Duffing oscillator are qualitative analysis, this paper uses Hamiltonian to construct statistic for quantitative decision. Hamiltonian can depict the dynamics in real time, and it can be used to do decision in short time situation. This paper utilizes mean pseudo Hamiltonian to depict the changing of the states of Duffing systems in real time. Our scheme can detect weak signal in lower signal-noise-ratio quickly. Simulation verifies the effectiveness of the proposed scheme.