Load balancing is important for distributed computing systems to achieve maximum resource utilization, and process migration is an efficient way to dynamically balance the load among multiple nodes. Due to limited capacity of a single cluster, it’s necessary to share the underutilized resources of other sites. This paper addresses the issues in multi-cluster load balancing based on process migration across separate clusters. Key technology and mechanisms are discussed and then the implementation of a prototype system is described in detail. Experimental results depict that by achieving multi-cluster load balance, surplus resources can be efficiently utilized and the makespan is also greatly reduced as a result.