The　7475　aluminum　alloy　backfill　friction　stir　spot　welding　(FSSW)　was　studied.　Firstly,　the　2　mm　7475　aluminum　alloy　was　welded　by　different　process　parameters,　then　the　surface　and　cross　section　of　the　joint　were　observed,　and　the　hardness　and　tensile　shear　test　of　the　sample　were　carried　out.　The　results　show　that　the　joint　can　be　divided　into　four　parts　according　to　the　weld　microstructure,　and　the　grain　size　of　different　parts　is　different.　Hook,　holes　and　unjointed　microstructures　due　to　poor　fluidity　of　the　material　are　found　in　the　microstructure.　At　the　same　time,　the　hardness　of　the　base　metal　zone　and　stirring　zone　is　significantly　higher　than　that　of　the　heat　affected　zone　and　the　thermo-mechanically　affected　zone;　in　addition,　the　highest　hardness　and　tensile　shear　strength　of　the　joint　vary　with　welding　tool　rotation　speed　and　the　depth　of　the　sleeve.　However,　the　effect　of　the　stirring　sleeve　movement　rate　on　the　mechanical　properties　of　the　solder　joint　is　not　obvious.