The invention discloses a power grid automatic voltage control method and system taking global optimum as a target. The method comprises the steps of obtaining a real-time state evaluation result of the power grid according to calculation cycles, if the state evaluation result is converged and telemetering percent of pass is greater than or equal to a threshold value, calculating and obtaining a state expectation value of each reactive voltage source based on a global optimization method, or otherwise, if the bus voltage exceeds standard, assigning the state expectation value of each disperse and adjustable reactive voltage source with the value of the last calculation cycle separately, and calculating and obtaining the state expectation value of each continuous and adjustable reactive voltage source; if the bus voltage dose not exceed standard, assigning the state expectation value of each reactive voltage source with the value of the last calculation cycle separately; finally issuing and executing the corresponding state expectation values; and the system comprises respective units corresponding to the steps in the abovementioned method. According to the method and the system, power grid partitioning is not needed, the voltage requirement of each bus of the power grid can be satisfied while the global reactive power flow optimum is taken as the control target, so that the network loss can be further lowered, and the power grid global optimal economic operation can be realized.