Tooth　crack　is　a　general　fault　causing　the　operating　failure　of　gearbox,　for　this　account,　the　effect　of　crack　on　the　meshing　stiffness　and　vibration　of　gearbox　are　simulated　in　this　paper　to　study　and　provide　some　theory　basis　for　fault　diagnosis.　Firstly,　the　working　process　of　cutting　tooth　is　put　in　use　to　calculate　the　precise　profile　in　theory　of　tooth,　then　the　meshing　stiffness　is　deduced　under　the　new　profile.　Secondly,　for　the　meshing　stiffness,　two　methods　calculating　the　effective　tooth　thickness　under　crack　is　introduced　and　compared　in　different　crack　depths.　Afterwards,　the　vibration　of　gearbox　is　extracted　by　putting　the　meshing　stiffness　in　a　6　degree　of　freedom　dynamic　model,　and　the　effect　of　crack　under　two　affecting　lines　on　the　gear　dynamics　is　expressed　by　processing　the　residual　signal　using　the　fault　detection　indicators.　This　work　aims　at　improving　the　dynamic　simulation　for　gearbox,　and　it　could　be　served　as　a　reference　for　monitoring　the　great　failure　event　in　gearbox.