The change of background's chaos feature when there are moving targets was analysed, and a new algorithm based on Lyapunov exponents of image block for moving targets detection on the sea surface, which is used for ship targets detection on the sea surface, was proposed. The new method defines the term of image gray level distance, extends the calculation method of Lyapunov exponents from single dimension signal to image signal based on the improved Wolf method, uses the change of the largest gray level distance Lyapunov exponents to detect ship targetssubmerged in chaos background. Experimental results show that the sea surface background has chaos feature. The proposed algorithm can detect ship targets effectively and can get better result than statistical analysis.