Although power systems are integrated， the voltage/reactive power control centers are based on independent calculation model lacking in mutual coordination， which may result in irrational distribution of reactive power or even control oscillation. Based on the decoupled network equations of active and reactive power and according to the decomposition and coordination idea of power networks， the interaction among interconnected power systems is derived when exercising regional voltage/reactive power control. And then， a coordinated control model considering the economical operation of both the local and interconnected power networks is obtained by using the optimal reactive power injection of interconnected power systems evaluated by the optimal matching injected flow. Simulations on two IEEE systems and an actual system have verified that the model proposed can accomplish coordinated optimal control of voltage/reactive power with little information exchange between interconnected power systems.