Pili are critical in host recognition， colonization and biofilm formation during bacterial infection. Here， we report the crystal structures of SafD- and SafD-SafA-SafA (SafDAA-) in Saf pili. Cell adherence assays show that SafD and SafA are both required for host recognition， suggesting a poly-adhesive mechanism for Saf pili. Moreover， the SafDAA- structure， as well as SAXS characterization， reveals an unexpected inter-molecular oligomerization， prompting the investigation of Saf-driven self-association in biofilm formation. The bead/cell aggregation and biofilm formation assays are used to demonstrate the novel function of Saf pili. Structure-based mutants targeting the inter-molecular hydrogen bonds and complementary architecture/surfaces in SafDAA- dimers significantly impaired the Saf self-association activity and biofilm formation. In summary， our results identify two novel functions of Saf pili: the poly-adhesive and self-associating activities. More importantly， Saf-Saf structures and functional characterizations help to define a pili-mediated inter-cellular oligomerizaiton mechanism for bacterial aggregation， colonization and ultimate biofilm formation.