The invention provides a method for controlling reactive voltage in an ultra-high voltage grid on the basis of improved economic voltage difference. For current simulation and problems in controlling the reactive voltage in the ultra-high voltage grid, the reactive voltage in the ultra-high voltage grid is balanced in real time by real-time coordination of low voltage side conventional reactive compensation with 500kV grid dynamic reactive compensation. The method allows real-time coordinated control for the low voltage side reactive compensation and 500kV grid side dynamic reactive compensation, and ensures real-time reactive power and efficient and economic operation of the ultra-high voltage grid. In addition, by the use of the method, operating frequency in conventional compensation switching can be reduced greatly, reactive loss of transformers is lowered further, and busbar voltage fluctuation caused by power fluctuation of tie lines is suppressed to a certain degree.