A weak signal is basically difficult to detect because its magnitude is small and it is often submerged in noised. A chaotic detection method for weak sine signals based on the cross-correlation and the Lyapunov exponent is given. Firstly， the cross-correlation method is applied to preliminarily remove noises， and then the chaotic detection method can be used to extract the weak sine signal. The proposed method adequately exerts the noise reduction ability of the cross-correlation method and the signal extraction ability of the chaotic detection method. Moreover， the proposed method uses Lyapunov exponent as the judgment criterion of chaotic dynamics. It can automatically recognize the critical state between the chaotic motion and the periodic motion and give the exact threshold value which is used to calculate the weak sine signal. Simulation results show that the proposed method can detect weak sine signals in strong noise， and the detection precision is superior to that of the cross-correlation method and the chaotic detection method.