A　summation-by-parts　simultaneous　approximation　term　(SBP-SAT)　finite-difference　time-domain　(FDTD)　subgridding　method　is　proposed　to　model　geometrically　fine　structures　in　this　article.　Compared　with　other　works,　the　proposed　SBP-SAT　FDTD　method　uses　the　staggered　Yee＇s　grid　without　adding　or　modifying　any　field　components　through　field　extrapolation　on　the　boundaries　to　make　the　discrete　operators　satisfy　the　SBP　property.　The　accuracy　of　extrapolation　keeps　consistency　with　that　of　the　second-order　finite-difference　scheme　near　the　boundaries.　In　addition,　the　SATs　are　used　to　weakly　enforce　the　tangential　boundary　conditions　between　multiple　mesh　blocks　with　different　mesh　sizes.　With　carefully　designed　interpolation　matrices　and　selected　free　parameters　of　the　SATs,　no　dissipation　occurs　in　the　whole　computational　domain.　Therefore,　its　long-time　stability　is　theoretically　guaranteed.　Four　numerical　examples　are　carried　out　to　validate　its　effectiveness.　Results　show　that　the　proposed　SBP-SAT　FDTD　subgridding　method　is　stable,　accurate,　efficient,　and　easy　to　implement　based　on　the　existing　FDTD　codes　with　only　a　few　modifications.