Migrating workflow is an effective resolution to solve the problems of incomplete specification of business process and frequent changes of environment in workflow management community. Communication mechanism between different instances is one of the most important components to enable effective negotiation and cooperation in an autonomous alliance. However, existing communication schemes are not suitable for the dynamic and prompt selforganization of migrating instance alliance. In this paper, a reliable asynchronous communication model based on existing forwarding proxy and mailbox methods is provided to deal with such problems. The model captures the primary features on configuration and task execution of migrating workflow and balances between reliability and performance of the system. A series of algorithms are also presented to support reliable and asynchronous communication, efficient location tracking and timely proxy update. Moreover, original simulations show that the model can ensure reliable inter-instances communication, and keep overheads at reasonably low.