The invention discloses an automatic voltage optimal control method of a power grid. The method comprises the steps of firstly only adjusting a continuous reactive power resource and performing three-level voltage control on the condition that a discrete reactive power resource expectation value is not adjusted and switch equipment does not act to achieve control targets such as a power grid voltage and economic running; secondly, continuing the discrete reactive power resource expectation value and performing three-level voltage control on a continuous reactive power resource if no practical solution exists or power loss is excessive high; thirdly, fixing the discrete reactive power resource expectation value after optimization, and performing three-level voltage control on the continuous reactive power resource; fourthly, obtaining a pivot bus voltage expectation value of each region after secondary optimization, and performing second-level voltage control; and finally obtaining each continuous reactive power resource expectation value. By the method, unified and coordinated optimization control of discrete and continuous reactive power resources is achieved, the continuous reactive power source is preferably considered to be adjusted so as to achieve the targets such as automatic voltage control and optimal economic running, and the action frequency of the switch equipment is effectively reduced; and moreover, the method is built on the basis of a traditional automatic voltage control (AVC) system and is convenient to implement.