A novel fast scanning method is proposed to further stabilize and fasten the construction of extension velocities in level set method. Based on the partial differential equations and scanning schemes, the proposed algorithm only needs our four times to sweep and simple operations to build an extension velocity in O(N) time, where N is the number of grid points. The extended velocities are continuous and preserve the signed distance function without need for re-initialization. Moreover, the fast scanning algorithm has no dependence on the construction of the signed distance function. At last, the presented classical examples show that the proposed approach is accurate, simple and efficient.