As　a　weak　link,　the　joint　surface　of　machine　tools　directly　affects　the　stiffness　and　machining　accuracy　of　the　entire　machine　tool.　However,　obtaining　the　distribution　of　contact　stress　is　a　key　problem　that　urgently　needs　to　be　solved　in　exploring　the　mechanical　properties　of　joint　surface　connections.　In　this　paper,　a　tangential　contact　stress　ultrasonic　testing　method　based　on　the　equivalent　medium　theory　is　proposed,　providing　data　support　for　accurately　characterizing　the　contact　state　of　the　joint　surface.　A　virtual　material　model　of　rough　contact　interface　based　on　fractal　theory　is　established　to　explore　the　coupling　relationship　between　the　propagation　speed　of　ultrasonic　critical　refracted　longitudinal　waves,　the　tested　object＇s　material　parameters,　and　stress.　Moreover,　the　expression　of　the　equivalent　material　parameters　of　the　ultrasonic　critical　refracted　longitudinal　wave　propagation　layer　with　stress　variation　is　derived.　An　ultrasonic　acoustic　time　difference　model　considering　changes　in　contact　surface　material　parameters　is　constructed　based　on　the　principle　of　acoustic　elasticity　of　ultrasound.　Furthermore,　a　critical　refractive　detection　model　for　longitudinal　wave＇s　tangential　contact　stress　is　established　based　on　equivalent　medium　theory.　Experimental　specimens　and　stress　detection　plans　are　designed,　and　the　tangential　contact　stress　of　the　specimens　is　measured　based　on　the　division　of　surface　grids.　The　equivalent　stress　value　of　each　node　is　calculated　based　on　the　von　Mises　stress　by　measuring　the　stress　in　the　X/Y　direction.　In　addition,　a　shear　stress　distribution　cloud　map　of　the　contact　surface　is　drawn.　Finally,　the　accuracy　of　the　detection　results　is　confirmed　by　combining　theory,　simulation,　and　experimental　methods.　The　proposed　tangential　contact　stress　detection　method　based　on　critical　refractive　longitudinal　waves　is　significant　for　lowstress　assemblies.