File migration is a key technology to improve the efficiency of cluster file systems. In this paper, we give a thermal model that shows the file access frequency at each node of a cluster. Based on this model, we introduce a file migration policy, which can balance the workload of clustering server nodes. It is used to decide the optimal place of a single file in the cluster environment under a heavy workload. And the results of the simulation comparison experiments show that using this policy can decrease the average file access response time of a 4, 8, or 16 node cluster at a rate of about 35.52%, 62.43% and 49.07%, respectively. Furthermore, if the nodes are a uniform number, migration on the application server end will lower the accelerated speed of the average response time, i.e., the heavier the total workload, the more efficient the policy. In general, this policy can efficiently reduce the average file access response time, give a good extendibility and enhance the performance of the cluster file system as a whole.