Single-atom　catalysts　(SACs)　have　attracted　intensive　attention　due　to　their　highest　atom　utilization,　especially　for　expensive　noble　metal　catalysts.　This　article　provides　a　comprehensive　review　of　defect　engineering　in　the　preparation　of　SACs.　A　unique　view　of　the　synergy　of　single　atoms　and　defects　during　biomass　conversion　reactions　is　presented.　The　role　of　defects　on　SACs　in　biomass　conversion　reactions　is　discussed.　The　adsorption　activation　mechanism,　local　promotion　mechanism,　active　site　renewal　mechanism,　and　selective　recognition　mechanism　of　defects　are　proposed　and　explained.　Through　the　classification　and　in-depth　analysis　of　typical　biomass　conversion　reactions　using　SACs,　the　promising　potential　of　defect　engineering　in　broader　biomass　upgrading　is　illustrated.　It　is　hoped　that　this　paper　provides　guidance　for　the　rational　design　of　SACs　and　their　application　in　a　wide　range　of　systematic　biomass　conversions.