Abstract: Based on the characters of Web and Peer-to-Peer（P2P） systems，an algorithm of auto-generating index of media files for P2P systems from Web is proposed.This method can efficiently solve the problem about lack of descriptors of media files，which causes the low recall and precision in key words searching.Besides，to satisfy the increasing information of Web and the changing popularities of people，an auto-updating strategy for peers is applied.From the experiments，it shows that this method can expand the describing information for media files and significantly improve the searching capacity of P2P systems.