Abstract: Soil moisture content is an important factor of influencing crop's growth. It is very important that the moisture content is measured online in water-saving irrigation control system especially. By application of computer digital image processing technique, a method for measuring soil moisture content online is put forward. After median filtering, image mode transforming, and "bad area" filtrating to images of soil layer, the authors extract the characteristic parameter of a image, that is gray-value, and carry out experiments on the relationship between the gray-value of soil layer image and the soil moisture content. The theoretical analysis and the experimental results show clearly that there is an approximate linear function relationship between the percentage of soil moisture content and gray-value of soil layer image.