As　one　of　the　most　ecologically　and　economically　important　ecosystems,　tidal　flats　are　under　severe　threat　from　extensive　coastal　reclamation.　The　lack　of　time-series　topographic　data　limits　the　understanding　of　the　evolution　of　tidal　flats　under　stress.　Existing　approaches　to　tidal　flat　topographic　mapping　are　complex　and　weak　in　their　representation　of　micro-geomorphic　features.　In　this　study,　we　propose　a　pixel-by-pixel　inundation-based　topographic　mapping　algorithm　(PITMA)　to　accurately　reconstruct　tidal　flat　topography　from　full-time-series　satellite　imagery　and　to　assess　multi-decadal　changes　in　the　tidal　flats　of　Fujian　Province,　which　has　undergone　intensive　coastal　reclamation.　The　results　show　that　the　proposed　algorithm　can　generate　accurate　tidal　flat　topography　with　an　RMSE　of　0.30　m　in　Fujian　province.　The　total　areas　of　tidal　flats　were　1311.27　km2,　1136.33　km2,　and　1048.18　km2　in　2000,　2010,　and　2020,　respectively.　Coastal　reclamation　in　Fujian　Province　has　resulted　in　the　loss　of　more　than　20%　of　the　tidal　flats,　but　the　average　elevation　of　the　tidal　flats　has　increased　by　40%　with　mean　elevations　of　-0.68m,　-0.82m,　and　-0.41m　in　2000,　2010,　and　2020,　respectively.　Meanwhile,　the　slope　of　tidal　flats　also　increased　in　most　tidal　flats　in　bay　areas.　Topographic　profiles　in　2000,　2010,　and　2020　showed　that　tidal　flats　in　bay　areas　gained　deposition　at　higher-elevation　areas　but　suffered　erosion　at　lower-elevation　areas,　due　to　insufficient　sediment　availability　and　extensive　land　reclamation,　then　the　tidal　flats　have　become　narrower　and　steeper.　In　contrast,　estuarine　tidal　flats　exhibited　seaward　expansion　with　stable　slopes　because　of　available　sediment　provision　from　the　upstream　river.　Such　tidal　flat　morphology　would　be　at　increased　risk　of　erosion　as　sea　level　rises　and　sediment　supply　decreases.　This　quantitative　assessment　of　the　impact　of　coastal　reclamation　on　tidal　flat　evolution　provides　scientific　guidance　for　sustainable　coastal　management.