Ultrasonic　welding,　known　for　its　severe　plastic　deformation,　faces　the　challenge　of　balancing　sufficient　deformation　at　the　welding　interface　with　minimizing　damage　to　the　substrate.　This　study　utilizes　the　anisotropic　deformation　mechanisms　and　mechanical　properties　of　nanotwinned　Cu　(nt-Cu).　Specifically,　Cu　coatings　featuring　nanotwin　layers　aligned　parallel　to　the　ultrasonic　vibration　direction　were　employed　as　interlayers　in　ultrasonic　welding　of　Cu-Cu　joints.　The　effects　of　the　nt-Cu　interlayer　on　the　welding　quality　and　the　deformation　mechanisms　under　the　various　welding　pressures　are　investigated.　Experimental　and　molecular　dynamics　simulations　demonstrate　that　at　low　welding　pressures,　the　nt-Cu　interlayer　undergoes　deformation　and　detwinning　primarily　through　twin　boundary　migration.　This　mechanism　effectively　mitigates　work　hardening　during　the　welding　process,　localizes　deformation　at　the　welding　interface,　and　significantly　enhances　the　strengths　of　the　Cu-Cu　joints.　The　maximum　enhancement　proportion　occurs　at　a　welding　pressure　of　8　psi,　up　to　26.75%　compared　to　conventional　coarse-grained　copper.　As　the　welding　pressure　increases,　the　strengthening　effect　gradually　weakens.　The　deformation　mechanism　of　nt-Cu　transitions　to　dislocation　transverse　and　threading.　The　interaction　between　dislocations　and　twin　boundaries　forms　incoherent　twin　boundaries　and　9R　phases,　resulting　in　work　hardening　of　the　interfacial　regions　and　reduction　of　the　strengthening　effect.