Flap surgery is to surgically remove part of the periodontal pocket and the wall of the pocket, and turn up the mucoperiosteal flap of the gingiva, scrape the subgingival tartar and granulation tissue under direct vision, trim the alveolar bone if necessary, and then reset and suture the gingival flap to eliminate the periodontal pocket, or make the pocket shallow. The surgical principle is to apply different surgical incisions to form periodontal tissue flaps of different thickness and shape, turn them up, expose the tissue in the lesion area, scrape the infected granulation tissue and root surface tartar thoroughly, trim or not the bone shape, and then reset the soft tissue flap after debridement to achieve the purpose of making the periodontal pocket shallower and promoting bone repair. Flap surgery is currently the most widely used periodontal surgery method, and is also the basis for many other procedures such as osteoplasty, bone grafting, guided tissue regeneration, etc. Its indications mainly include: 1, deep periodontal pockets or complex periodontal pockets, the periodontal pockets are still above 5mm after basic treatment, and bleeding after probing. 2, the bottom of the periodontal pocket exceeds the membrane gingival joint boundary, and it is not suitable for periodontal pocket removal. 3, there is a bone pocket formation, need to make bone repair or bone grafting. 4, root bifurcation lesion with deep periodontal pockets or a periodontal pulp joint lesion patients, need to level the root surface under direct vision, and expose the root bifurcation, or need to amputate a certain affected root.