The small-world network theory， which studies the intrinsic spreading mechanism of the large-scale chain-like fault in a power grid， attaches great significance to the inherent vulnerability of the power grid itself and its pertinent enhancement measures. On this basis， an integrated algorithm for vulnerability assessment based on the small-world topological model is proposed. And the independent algorithm for each part， including the topological modeling of the bulk power grid， the statistics of basic characteristic parameters as well as fault simulations and the vulnerability assessment and so on， is then designed separately， thus forms an integrated algorithm with good expansibility and the ability to provide convenience to a through study of the structure vulnerability of a bulk power grid itself and large-scale chain-like fault mechanism. The proposed algorithm has been verified with good results by several actual power grids.