CV model is difficult to precisely segment the object which is partially occluded or is similar in gray value with the background or has obvious textures. In this paper, we add shape restraint equations of prior shape to the level set function, which keeps the curve to be a specific class shape during the whole evolvement and realizes shape preserving in object segmentation. Using the proposed model, we built the energy function for rectangle object, deduce a group of corresponding Euler-Lagrange ordinary differential functions, and evolve the level set function. By evolution, rectangle object can be properly segmented, and the final level set function is convicted just the quantitative description of the rectangle object. At the end of the paper, three groups of experimental results validate that the proposed model can correctly segment the rectangle object from complex backgrounds with lessened calculation and strong robustness.