In order to overcome the overconvergence and the premature convergence of lip boundary caused by the level set model for geometric shape detection, an improved level set model based on the prior shape （ PS-Level Set） is proposed. In this model, the prior shape information of lip is incorporated into an improved differential energy function, and the differences between the evolution shape curve and the prior shape curve are repeatedly compared during the curve-evolving process, which enables the evolution shape to gradually approach the prior one and to converge to the target object more accurately. Experimental results show that, as compared with the conventional level set model, the proposed model improves the detection accuracy by 8.38%.