This paper presents a distributed optimization method to solve the optimal reactive power flow (ORPF) problem for multi-area interconnected power grid. A simplified external network equivalence method is used to implement the regional ORPF calculation of each sub-grid, and the inter-area coupling constraint is transformed into equality constraints of boundary bus as injection power modified equation afterwards. Therefore, the distributed solution of multi-area ORPF problem can be realized through the update and transfer process of equivalent injection power and its corresponding boundary bus voltage vector. The proposed method omits the tedious process of network equivalence and reduces the burden of coordination layer, which accord with the local feature of reactive power optimization and control properly. The numerical simulation results of several IEEE cases show that the proposed method is effective and suitable for online application.