Emerging non-volatile, byte-addressable memories like phase-change memory can make data persistent at main memory level instead of storage. Since the read/write latency of Non-Volatile Memory (NVM) is very low, the overhead of software in a NVM system has become the main factor in determining the performance of the entire persistent memory system. Pmfs is a file system specifically designed for NVM. However, it still has an undesirable characteristic:each directory operation (create, open or delete) of Pmfs requires a linear search of the entire directory files, resulting in a cost linearly increased with the number of files in the directory. The performance of Pmfs under various workloads was evaluated and the test showed that the overhead of the directory operations had become the bottleneck of the whole system in some circumstance of particular workloads. To solve this problem, a persistent directory entry index was implemented in Pmfs to speed up directory operations. The experimental results show that under a single directory with 100 000 files, the file creation speed is increased by 12 times, the bandwidth is improved by 27.3%.