Line fault simulation which covers the whole network should be considered in actual large-scale power grid sag assessment. The calculation speed and stability of the simulation are the main factors that restrict its practicability. Therefore, this paper proposes a voltage sag assessment method for a large-scale power grid based on a multi-Agent system. This method constructs the multi-Agent system and formulates collaborative interaction rules according to simulation requirements. First, the simulation task is generated independently by the grid analysis and a fault setting Agent. Second, massive simulation is dynamically executed by simulation management and a computational Agent. Finally, the simulation data are stored asynchronously by the Agent and the evaluation results are generated. At present, this method has been applied to the voltage sag assessment of a provincial power grid, and good results have been achieved. Examples show that this method has high acceleration ratio and stable operation, and can provide technical support for the practical application of voltage sag assessment of a large power grid.