The invention belongs to the digital image processing technical field, in particular to a method for identifying content in colour digital image. The invention includes an image copy detection algorithm and a pornography identification algorithm. The image copy detection algorithm adopts cascaded method, and firstly comparison of image second order entropy is used for reducing the range in image library. Then SSIM algorithm is used for obtaining similarity in the reduced range. The pornography identification algorithm includes four weak graders, a skin region proportion grader, a face quantity grader, an invariant moment grader and a skin region distribution grader concretely; the four graders output four confidence coefficients, so as to form a four-dimensional characteristic vector, and the vector is input into a strong grader, thus obtaining final result. The higher the result of the strong grader is, the bigger the probability that the content is pornographic is. The system and method has wide application value in the Internet management and control field.