The distributed power system analysis becomes more and more important since the power grid is interconnected but its operation and control is decentralized. A novel asynchronous iterative distributed power flow algorithm based on the boundary bus injection power is proposed. Different with Ward equivalent model， a reduced network equivalent model is used. Therefore the process of network reduction and equivalence are avoided and the dependence on the coordination layer is reduced. The asynchronous iteration mode is used and the boundary bus injection power is updated by exchanging and combining the voltage phasors of the boundary buses. The numerical results on IEEE 9-bus， IEEE 14-bus， IEEE 39-bus and IEEE 118-bus test systems show that the proposed method is effective， well-convergent and suitable for on-line application.