SAR(synthetic aperture radar) remote sensing has the capabilities of 24hour and allweather observation and the ability to penetrate the terrain surface. Especially, with the recent development of new imaging radar technologies, SAR has become a useful and efficient method on monitoring the water surface information changing and on investigating water resources. By using multitemporal ASAR data, this paper proposes a fast water surface information changing detection approach integrating logratio operator and KI algorithm. Firstly, after removing the speckle noise of original image data, the logratio operator is used to represent the image information as changed and unchanged classes. Secondly, by using KI algorithm, the threshold between these two classes can be detected quickly and exactly, then the changing information of water surface obtained automatically. Finally, the result of extraction of water surface information changing is evaluated by a method of comparison. Experimental results on ASAR data prove the effectiveness of the proposed automatic method in detecting the change information of water surface and the approach in this paper is approved to be an effective method.