In　this　paper,　the　rod　model　is　adopted　to　describe　the　flexible　fiber　in　the　simple　shear　flow.　By　using　fibers　with　large　stiffness,　the　Jeffery　orbits　are　obtained　to　validate　the　model.　Then　the　effect　of　the　concentration　on　the　dynamic　behavior　of　flexible　fibers　in　semi-concentrated　suspension　is　numerically　simulated.　The　results　show　that　the　fibers　bend　and　twist　during　the　motion　while　those　shape　appear　in　different　form,　including　springy　motion,　snakelike　motion,　coiled　motion.　Meanwhile,　the　collision　and　friction　between　fibers　play　an　important　role　in　the　evolution　of　bending　angle,　while　their　effect　on　the　torsion　angle　is　relatively　small.　The　value　and　fluctuation　of　the　bending　angle　increase　with　the　increasing　number　of　collisions.　In　addition,　it　is　also　found　that　instead　of　hydrodynamic　impacts,　the　dynamic　characters　of　fiber　are　actually　mainly　influenced　by　the　collision　and　friction　between　fibers,　as　well　as　deformation　of　fibers.　Thus,　the　fiber　orientation　will　be　more　and　more　random.