Recent advances in computing and communication technologies have made VOD （video-on-demand） technically feasible and economically. The VOD system needs massive storage server because of the characteristic of the continuous media. Hierarchical storage is a good solution for the system under low cost. In this article, the authors first propose a hierarchical architecture model and the concept of disk cache, then develop a replacement algorithm based on access frequency by the model, finally analyse and simulate the algorithm. The results show that the algorithm avoids cache pollution problem in LFU (least frequently used) algorithm and well suits the applications of the continuous media data objects.