It is necessary to understand the mode of I/O mode before designing a parallel file system (PFS). The structure, interface and algorithm of cache/prefetch is closely related to application. Now, most analyses are based on static I/O model, but the dynamic mode is much more important to PFS. This paper analyses the dynamic I/O mode of parallel scientific application, and summarizes the characteristics of I/O size, interval, sequence and so on. The conclusion of what should be considered when designing a PFS is finally drawn.