This　article　studies　the　causes　of　the　time-varying　stiffness　of　harmonic　drive　gear　and　the　number　of　meshing　teeth　and　the　meshing　length　under　different　torques.　Multi-pair　tooth　meshing　characterizes　the　operating　process　of　the　harmonic　gear　reducer.　However,　the　change　of　the　number　of　meshing　pairs　is　still　not　clear.　Therefore,　the　mechanism　of　the　time-varying　stiffness　of　harmonic　drive　gear　was　studied　using　both　the　theoretical　approach　and　simulation　method,　and　the　mathematical　model　of　the　time-varying　stiffness　of　harmonic　gear　reducer　was　established.　Moreover,　a　method　for　calculating　the　torque　under　different　stiffness　values　was　proposed.　The　relationship　between　varying　torque　and　time-varying　stiffness　was　established　by　the　number　of　meshing　teeth　pairs.　The　established　time-varying　stiffness　model　and　the　proposed　method　are　feasible,　and　the　effectiveness　was　proved　through　experimental　research.　This　study　provides　theoretical　support　for　the　analysis　of　load　distribution　in　the　flexible　wheel　by　the　stress-strain　loads　and　related　problems.