This　work　for　the　first　time　reports　on　a　simple　and　rapid　colorimetric　immunoassay　with　rapid　coordination　of　ascorbic　acid　2-phosphate　(AAP)　and　iron　(III)　for　determination　of　carcinoembryonic　antigen　(CEA,　used　as　a　model)　by　using　Fe2O3　nanoparticle　based-chromogenic　substrate　system.　The　signal　was　produced　rapidly　(1　min)　from　the　coordination　of　AAP　and　iron　(III)　with　color　development　of　colorless　to　brown.　TD-DFT　calculation　methods　were　employed　to　simulate　the　UV-Vis　spectra　of　AAP-Fe2+　and　AAP-Fe3+　complexes.　Moreover,　Fe2O3　nanoparticle　could　be　dissolved　with　the　aid　of　acid,　thereby　releasing　free　iron　(III).　Herein,　a　sandwich-type　immunoassay　was　established　based　on　Fe2O3　nanoparticle　as　labels.　As　target　CEA　concentration　increased,　the　number　of　Fe2O3　labelled-antibodies　(bound　specifically)　increased,　resulting　in　loading　more　Fe2O3　nanoparticle　on　platform.　The　absorbance　increased　as　the　number　of　free　iron　(III),　derived　from　Fe2O3　nanoparticle,　increased.　So,　the　absorbance　of　reaction　solution　is　positively　correlated　with　antigen　concentration.　Under　optimal　conditions,　the　current　results　showed　good　performance　for　CEA　detection　in　the　range　0.02-10.0　ng/mL　with　a　detection　limit　of　11　pg/mL.　Moreover,　the　repeatability,　stability,　and　selectivity　of　the　colorimetric　immunoassay　were　also　acceptable.