The invention discloses a reactive voltage optimization control method and system for an extra-high voltage alternating current transformer substation, and belongs to the technical field of power systems and automation thereof. According to the method, the reactive exchange capacity at the 1000kv side and the 500kv side of the extra-high voltage transformer substation is used as an important basisfor controlling the reactive voltage of the extra-high voltage transformer substation, and the reactive voltage regulation is performed on the voltage out-of-limit transformer substations one by oneby starting from the overall reactive voltage control of the extra-high voltage power grid, so that the reactive exchange capacity at the 1000kv side and the 500kv side of the extra-high voltage transformer substation is minimum on the premise of ensuring that the voltage of each extra-high voltage transformer substation is operated in the voltage limit range, the utilization rate of devices suchas transformers is improved, the active loss of the extra-high voltage power grid line is reduced, and the safe and stable economic operation of the extra-high voltage power grid is ensured.