Abstract: In P2P file system, writing file is inefficient because a large number of copies and high network delay. In this paper, a parallel writing mechanism based on core copy set (PWMBCS) is proposed to solve those problems. Paralleling writing is given out to deal with network delay. The control algorithm for write order and parallel degree also is given out in this paper. As shown in many tests, the core copy set can form with writing, and PWMBCS is six times more than normal writing mechanism.