Considering the measurement system for the suspended position of the transparent film in the water, an image processing method based on watershed transform for contour detection has been proposed. The difference between the film and water has been researched by obtaining the image information through the image sensor under special light source. An algorithm of watershed which uses minima imposition to solve the problem of over segmentation for the detection is applied. Depending upon the improved algorithm, which aims to obtain the contour of the film, the target areas in the image integrate together. The result of the position is gained by the image processing technology, which provides the technical basis for automatic operation of catching film in the water.