The invention relates to the technical field of automatic voltage control in an electric system, in particular to a relaxation reactive voltage optimizing system based on province-district coordination and a working method of the relaxation reactive voltage optimizing system. At the present stage, local areas are administered by a dispatcher control center, the voltage regulation capacity can not be used sufficiently, and even voltage quality is affected because the regulation effects among the areas are opposite. The relaxation reactive voltage optimizing system comprises an obtaining module, a calculating module, a sending module, a processing module and a receiving module, wherein the obtaining module is used for obtaining a power grid model and real-time data; the calculating module is used for performing state estimation on collected data and performing relaxation voltage reactive power optimization calculation to obtain a coordination strategy; the sending module is used for sending the coordination strategy to a district dispatcher; according to the processing module, the coordination strategy is received by the district dispatcher and then processed to control actions, and if a province dispatcher is needed for matching, the upper and lower bounds of expected voltage are reported; according to the receiving module, a voltage coordinating request of the district dispatcher is received by the province dispatcher and serves as the optimization constraint of next circulation. According to the technical scheme, on the premise of ensuring qualified voltage, regulating frequency of a discrete device is reduced, network loss is effectively reduced, and convergence reliability of an optimization algorithm is improved.