Abstract: A priori shape parametric active contour model has been widely used in image segmentation because of their computational efficiency and stability. The model can deal with properly the concave regions in the image and provide an accurate segmentation to weak edges and noise images. An initial contour must not be set near the feature of the image and can provide a large capture range. Priori shape force field incorporates into an active contour model. The novel model can avoid computing the distance of priori shape contour to active contour, and can decrease the complexity. The model solves efficiently some drawbacks of a parametric active contour model. Experiments demonstrate that the model curve is driven to the object boundary by the new forces even if the initial contour does not close the true boundary and the object is nonconvex or has weak edges and noise on medical CT and ultrasound images.