Distributed power flow algorithm for EMS application of multi-control-centers based on asynchronous iteration is theoretically studied in this paper. Based on online tracing of Thevenin equivalence for exterior-network， a fixed-point iteration scheme for boundary state variable is designed and an asynchronous iteration algorithm is proposed. This algorithm can follow the variation of system state and give a consistent power flow result with the whole power system calculation. This algorithm is implemented by three steps: real-time equivalence， asynchronous iteration initialization and asynchronous iteration. Agent mechanism for coordination layer is established and applied to the three algorithm steps and a complete asynchronous iteration is realized. Some measures are taken to ensure that the accustomed modeling methods for sub-systems are still usable. It is free for any sub-system to join in or quit from the distributed calculation. So， this method is very suitable for the application of distributed EMS under a poor communication condition between control centers. The convergence performance of the algorithm is testified by digital simulations on IEEE 118-bus system and a real system.