The detection and identification of the ship target on the sea surface is significant for the monitoring and the target strike. The weak ship target with less texture information is affected by the sea surface shadow and noise interference, which results in the poor detection results with common algorithms. The weak ship target detection algorithm on sea surface is proposed based on the channel separation and minus extend contrast sensitivity function. Firstly, the multi-resolution image scale pyramid of the pixel intensity channel and the noise-edge channel is constructed. Secondly, the minus extend contrast sensitivity function under different scale space is established, and the weight of each position is modulated. Finally, based on the weighted spatial scale coefficient, the two channels' saliency maps are obtained. The weak ship target detection of noisy image is realized by channel differential processing. The experimental results show that compared with the other five algorithms, the proposed algorithm has high detection precision (97.30%), recall (84.71%) and F-Score (94.49%), and has strong anti-noise ability which is suitable to the weak ship target detection of noisy optical remote sensing image on sea surface.