In high voltage distribution network system, combining with the sensitive load characteristics, this paper introduces risk of voltage sag as stability considerations object. It selects STATCOM device to function as a dynamic reactive power compensation and researches transient voltage stability. Single objective function model of reactive power optimization is put forward, which quantitatively adds the loss of voltage sag, the line active loss and cost of dynamic reactive power compensation devices. Here we calculate the multiple sets of the total cost corresponding to reactive power capacity. Finally, the fitting curve of cost and capacity is given by using MATLAB software to seek the corresponding optimal solution in some area of Guangdong power grid and the result shows that it not only improves the safety and reliability of the power system but also greatly reduces the economic cost.