With the development of smart grid construction and operation, the appearance of clean energy power generation system access to load center and the formation of the regional interconnected power grid have increased the probability of power failure in large areas, so the secure operation of load nodes deserves special attention. In this paper, it puts forward the state vulnerability assessment model and index based on the voltage stability margin, considering the power access to load node, which make the state vulnerability assessment system perfect. Based on state vulnerability index results, the state assessment model of grid is established to judge the optimal access. Final, test results on IEEE 39-node system illustrate that the algorithm is effective and practical. In this way a theoretical reference is provided for the further study on reasonable planning power network and dispatching.