The　torsional　stiffness　of　the　reducer　is　commonly　assumed　to　be　static　and　tested　using　the　hysteresis　curve　method.　However,　practical　engineering　experience　shows　that　the　torsional　stiffness　is　dynamic　rather　than　static,　and　the　hysteresis　curve　method　cannot　accurately　represent　the　reducer＇s　torsional　stiffness　during　actual　operation.　To　address　these　issues,　this　study　conducts　theoretical　research　on　the　dynamic　characteristics　of　torsional　stiffness,　introduces　a　new　concept　called　dynamic　torsional　stiffness,　and　provides　its　analytical　formula.　A　dynamic　torsional　stiffness　testing　method　based　on　the　transmission　error　method　is　proposed,　and　its　feasibility　and　consistency　with　the　hysteresis　curve　method　are　theoretically　demonstrated.　This　method　allows　the　torsional　stiffness　information　of　the　reducer　to　be　obtained　under　all　operating　conditions.　Through　experimental　research,　the　limitations　of　the　hysteresis　curve　method　are　explained,　and　the　dynamic　characteristics　of　the　reducer＇s　torsional　stiffness,　along　with　the　effectiveness　of　the　transmission　error　method,　are　verified.　Consequently,　the　torsional　stiffness　information　of　the　reducer　under　all　operating　conditions　is　obtained.　In　conclusion,　this　method　is　valuable　for　both　theoretical　and　engineering　applications,　as　it　offers　a　more　accurate　representation　of　the　dynamic　torsional　stiffness　of　the　reducer　in　real-world　scenarios.