A　rough　surface-based　three-dimensional　contact　stiffness　calculation　method　for　a　spur　gear.　The　method　comprises　:　　carrying　out　hexa　meshing　under　the　requirement　that　all　square　meshes　on　a　tooth　surface　are　equal　in　area;　　calculating　contact　pressure　distribution　of　a　smooth　tooth　surface　on　the　basis　of　a　finite　element　method,　　and　extracting　the　pressure　of　a　node　in　a　contact　region;　　calculating　normal　contact　stiffness　and　tangential　contact　stiffness　of　a　single　square　mesh　on　the　basis　of　a　fractal　theory;　　and　calculating　the　contact　stiffness　of　the　tooth　surface.　The　method　further　improves　the　precision　of　a　gear　contact　stiffness　calculation　model,　　overcomes　the　defect　of　smooth　contact　based　on　a　Hertz　theory　in　a　previous　calculation　process,　　and　reveals　the　influence　rule　of　a　roughness　parameter　on　a　stiffness　characteristic　of　a　gear　in　the　process　of　contact　of　rough　tooth　surfaces　and　a　calculation　method.　Moreover,　　the　finite　element-based　three-dimensional　contact　stiffness　calculation　model　is　more　accurate　than　a　previous　two-dimensional　calculation　model　or　calculation　formula.　In　addition,　　the　influence　of　errors　in　actual　assembling　and　manufacturing　processes　of　a　gear　is　considered,　　so　that　the　contact　stiffness　of　the　gear　is　more　accurate,　　and　a　foundation　is　laid　for　dynamics　analysis　of　the　gear.