Water level monitoring is of great significance to the comprehensive management of water resources and navigation. With the development of machine vision technology, the image water level method is expected to replace observing the water gauge. Firstly, the history and the situation of water level detection are briefly described. The advantages of image method are illustrated by comparing various water level detection methods Then, Three key steps of image water level detection are analyzed: water gauge segmentation and distortion correction, water level detection and water level conversion. The methods and development of each step for each step are summarized at home and abroad. Finally, the problems of image water level detection at present are summarized and the future direction is prospected.