In this paper, we proposed a new active model to segment a given image, based on techniques of curve evolution and an energy minimization function. We use the semi-implicit scheme to implement the active model, instead of the classical explicit scheme, which is unconditionally stable and does not suffer from any time step size restriction. Finally, we present the algorithm and experimental results based on the semi-implicit scheme. Experimental results show that one can obtains a high quality edge contour.