To address the issue of the sub-bottom profile layer extraction in complex circumstance, this paper proposes a new demarcating method based on constraint of image information entropy. Firstly, the image of sub-bottom is divided into different blocks; then, the information entropy in each block is calculated and a relation model of information entropy and significant parameters are established according to drilling data; finally, the whole sub-bottom profiling is demarcated according to the model. It is revealed that this method has overcome the shortcomings of existing methods, realized the self-adapting and exacted demarcation of sub-bottom layers. The experiment has gained the same accuracy as the depth and thickness of layers got by drilling data.