This paper introduce a method of how to use remote sensing image to define arbitrary tidal line during tidal flat evolution. This method includes how to use graphic method or calculate method to get tidal place correction, which according to the location of timely water place line and combine it with the elevation data of water place line and another tidal place nearby it, in order to fix the place of tidal line. Using this method to investigate tidal flat evolution in Hainan Yangpu district have achieved good result .