The invention relates to a global reactive voltage optimization method for a power transmission and distribution network. The method includes the steps that coordinate boundary points of a power transmission network and an active power distribution network are selected; a power transmission network reactive voltage optimization model including complementary constraints is established, and an interior point method is used for solving; the optimization voltage of the power transmission network is converted into a three-phase voltage which is used as the three-phase voltage of an active power distribution network root node; a three-phase reactive voltage optimization model including complementary constraints of the active power distribution network is established, and the interior point method is used for solving; the three-phase optimization power of the active power distribution network is summated and then is used as the corresponding load power of the power transmission network; reactive voltage optimization of the power transmission network and reactive voltage optimization of the active power distribution network are in alternate iteration, when every two adjacent iteration changes of the boundary point power of the power transmission and distribution network meet convergence precision, the global reactive voltage of the power transmission and distribution network is subjected to optimizing convergence, and accordingly a global reactive voltage optimizing strategy is obtained. The problem that the reactive voltage optimization results of the power transmission and distribution network are inconsistent at the boundary points of the power transmission and distribution network can be solved, voltage and power mismatching at the boundary points is eliminated, and reactive resources of the two stages of networks are optimized.