As novel electric power sensitive loads are more sensitive to voltage change, voltage sag may bring huge economic loss to the normal work of new loads, resulting in severe reduction in the economic efficiency of the power grid. The present optimal reactive power compensation allocation methods mainly focus on the safe reliability of system, but the economic value affected by the voltage sag is insufficiently considered. This paper proposes an optimal allocation method which considers the huge economic loss of voltage sags and focuses on return on investment index of high-cost STATCOM. Firstly, based on quality engineering theory, voltage sag severity index and voltage sag economic loss index are proposed. Secondly, by comparing the voltage sag economic losses before and after the installation of STATCOM, this paper puts forward STATCOM return on investment index to assess the economic benefit of different installation capacity. Finally, based on above economic benefit evaluation indexes, an optimal reactive power compensation allocation method is proposed. The proposed method can both ensure the safe reliability of system and achieve the optimum economic benefit. A real power grid of Hunan is simulated using BPA. The simulation results verify the validation of the proposed method.