The invention relates to a coordinating voltage control method of a provincial power network and regional power networks based on two-way coordination constraint, belonging to the technique field of the automatic voltage control of an electric system. The method comprises the following steps: selecting the high-voltage side reactive power of a transformer of a transformer substation positioned on the boundary position of the provincial power network and each regional power network as the coordination variable of the provincial power network for the regional power networks; selecting a high-voltage side busbar voltage amplitude value of each transformer substation positioned on the boundary position of the current periodic provincial power network and the regional power networks as the coordination variable of the regional power networks for the provincial power network; establishing and solving an optimal power flow model of the provincial power network on a provincial power network side so as to obtain the state quantity of each node inside the provincial power network as well as the upper limit and the lower limit of the coordination constraint of the high-voltage side reactive power of the transformer of the transformer substation positioned on the boundary position; and substituting a coordination constraint condition of one control period of the regional power networks with a related constraint condition of the high-voltage side reactive power of the transformer of the transformer substation positioned on the original boundary position by each regional power network control center, and controlling the reactive power voltage of the region. The coordinating voltage control method can effectively reduce the network loss and enhance the voltage safety level of the power networks.