The connection of ultra-high voltage（UHV）transmission line has brought a challenge to the voltage operation and control of its neighboring power system.Facing engineering practice，this paper proposes an automatic voltage control（AVC）method for the UHV neighboring grids.The reactive voltage operation and control characteristics of UHV neighboring grids are analyzed，including the tremendous impact of UHV active power flow change on the bus voltage in UHV neighboring grids，the large voltage fluctuation，and the tight coupling among power plants and substations，etc.The smart voltage optimal control technology is proposed and implemented by taking into consideration these characteristics and the coordinated control among tightly coupled power plants and substations in the UHV neighboring grids.This method can reduce the irrational reactive power flow and bypass the unnecessary discrete device action，while ensuring voltage security.The proposed method is put into closed-loop field operation in China for its validated efficiency and practicability.