It is of great significance to quickly acquire spatiotemporal change in the information of waterline of remote sensing image. The extraction of the waterline of tidal flat on the remote sensing image has always been a difficult problem in the application of remote sensing technology. There are unique spatial relationships and spectral characteristics on the remote sensing image of waterline. The research area is the Chongming Dongtan of the Changjiang River Estuary. By integrating methods of color model transformation, information entropy calculation, maximum variance and edge detection, we explored how to enhance the contrast of land and sea on the Landsat-5 satellite image, and the edge extraction at different scales was studied. The calculation method of the spatial and spectral characteristics of the waterline using the thermal infrared band was given. A fast extraction method of waterline of sensory image taking the spatial relationship and spectral characteristics into account under the framework of object-oriented technology was proposed. Results show that: (1) The local threshold segmentation method based on the maximum between-class variance method can automatically extract the waterline of band 6. The waterline is continuous, complete, and rich in spatial information. (2) The combination of the optimum index factor method, the dispersion method and the color model transformation method can effectively enhance the contrast between land and sea. The local adaptive Canny operator based on the maximum between-class variance method can automatically detect the high precision edge of the enhanced remote sensing image. (3) Using the spatial relationship and spectral characteristic of waterline, the computer can recognize and connect waterline automatically. (4) The waterline extraction method proposed in this paper is fast and automated, inheriting strong continuity of the threshold segmentation method and high positioning accuracy and strong ability to present details of Canny operator. The results have significant value for researches on the dynamic changes in the coastal zone, the mechanism of land-sea interaction, the protection and development of coastal zone resources, and offshore engineering management.