To　reveal　the　internal　mechanism　of　the　radial　clearance　between　the　inner　and　outer　tubes　on　the　microstructure　of　the　electromagnetic　pulse　welding　joint　interface,　the　formation　and　evolution　mechanism　of　interface　microstructure　are　studied　by　changing　the　radial　clearance　between　the　304L　stainless　steel　and　T2　copper　tube　combined　with　microscopic　characterization　and　numerical　simulation.　The　study　shows　that　the　interface　presents　a　wave　shape,　which　is　due　to　the　Kelvin-Helmholtz　interfacial　instability　effect　caused　by　the　high-speed　collision.　As　the　radial　clearance　increases　from　1.65　mm　to　2.15　mm,　magnetic　intensity　maximum,　impact　speed,　impact　angle,　the　length　of　the　wavy　unbonded　region　and　the　wavelength　of　the　wavy　interface　increase.　Meanwhile,　the　stress　distribution　appears　S　shape.　The　interface　element　distribution　is　a　combination　of　local　high-temperature　melting　and　element　interdiffusion　at　less　high-temperature.　And　the　formation　of　the　steel-copper　electromagnetic　pulse　welding　joint　includes　the　arcing　of　upper　surface　of　the　outer　tube,　the　fragmentation　of　the　end　of　the　outer　tube,　the　generation　of　unbonded　areas　and　the　generation　of　bonded　areas.　The　reason　is　that　the　outer　tube　undergoes　different　plastic　deformation　under　the　combined　effect　of　the　propagation　direction,　the　sound　wave,　and　the　reflected　wave　of　the　shock　wave.　©　2023　Editorial　Office　of　Chinese　Journal　of　Mechanical　Engineering.　All　rights　reserved.