The　7475　aluminum　alloy　has　excellent　mechanical　properties　and　is　widely　used　in　the　aerospace　industry.　The　use　of　refill　friction　stir　spot　welding　(FSSW)　for　welding　can　effectively　eliminate　the　keyhole　in　the　traditional　friction　stir　spot　welding　head,　so　that　the　mechanical　properties　are　greatly　improved.　However,　when　the　refill　FSSW　was　performed　on　a　7000　series　high-strength　aluminum　alloy,　the　mechanical　properties　were　still　poor,　and　a　good　solder　joint　was　not　obtained.　When　refill　FSSW　was　applied　to　a　high-strength　aluminum　alloy,　more　heat　input　was　required　to　form　plastic　material　from　the　stirring　of　the　base　material.　When　the　welding　heat　input　is　high,　cracks　and　holes　were　easily　generated　in　the　joint.　Such　defects　affect　the　mechanical　properties　of　the　joint.　In　this　study,　the　morphological　characteristics　of　chain　pores　were　analyzed　by　laser　confocal　microscopy　and　scanning　electron　microscopy　(SEM),　and　the　causes　of　chain　holes　analyzed.　At　the　same　time,　a　series　of　process　parameter　experiments　were　carried　out　to　study　the　influence　of　process　parameters　on　the　chain　holes　in　the　solder　joints.　The　results　showed　that　the　7475　aluminum　alloy　refilled　FSSW　solder　joints　have　symmetric　distribution　and　chain　holes　in　the　SZ　region　with　higher　heat　input.　The　direction　of　the　chain　hole　was　consistent　with　the　flow　direction　of　the　plastic　material　during　the　welding　process.　The　existing　intermetallic　compound　(IMC)　in　the　base　metal　was　crushed　by　the　welding　tool　and　aggregated　in　the　SZ　along　the　flow　direction　of　the　material　due　to　the　stirring　of　the　welding　tool　and　continuous　improvement　of　the　welding　temperature　during　the　welding　process.　When　the　temperature　in　SZ　reaches　the　eutectic　temperature,　IMC　reacts　with　Al　to　cause　melting.　In　addition,　as　the　depression　depth　increases,　the　density　of　the　chain　holes　increases.　As　the　depressing　velocity　of　the　tools　increase,　the　density　of　the　chain　holes　decreased.　As　the　rotation　speed　increases,　the　chain　hole　density　increases.　©　2020,　Editorial　Board　of　Journal　of　Tianjin　University(Science　and　Technology).　All　right　reserved.