Many　studies　have　demonstrated　that　the　extracellular　domain　of　human　epidermal　growth　factor　receptor　2　(HER2　ECD)　level　in　serum　can　act　as　a　breast　cancer　biomarker　and　serve　as　a　monitoring　neoadjuvant　therapy　of　breast　cancer.　In　this　study,　we　developed　a　sensitive　ascorbic　acid　(AA)-mediated　AuNBPs　(gold　nanobipyramids)　growth　method　with　NADH　(reduced　nicotinamide　adenine　dinucleotide　I)　assistance,　and　we　further　fabricated　a　high-resolution　multicolor　immunosensor　for　sensitive　visual　detection　of　HER2　ECD　in　serum　by　using　AuNBPs　as　signal　and　antibody　as　recognition　probe.　The　NADH-assisted　AA-mediated　method　effectively　suppressed　color　formation　in　the　blank　and　greatly　improved　the　sensitivity　of　mediating　AuNBPs　growth,　allowing　us　to　use　a　low　concentration　of　AA　to　mediate　AuNBPs　growth　to　generate　more　colorful　and　clearer　color　changes.　The　proposed　multicolor　immunosensor　has　higher　resolution　and　more　color　changes　corresponding　to　HER2　ECD　concentrations.　It　can　be　used　to　detect　as　low　as　0.5　ng/mL　of　HER2　ECD　by　bare　eye　observation　and　0.05　ng/mL　of　HER2　ECD　by　UV-visible　spectrophotometry.　Using　the　immunosensor,　we　have　successfully　detected　HER2　ECD　in　human　serum　with　a　recovery　of　94%-96%　and　an　RSD　(n　=　5)　＜　5%.　The　results　obtained　with　our　immunosensor　were　consistent　with　those　obtained　with　ELISA,　verifying　the　immunosensor　has　good　accuracy.　The　immunosensor　exhibited　a　vivid　multicolor　change,　has　low　visual　detection　limit,　excellent　specificity　and　reproducibility,　and　robust　resistance　to　matrix.　All　the　above　features　makes　our　immunosensor　a　promising　assay　for　the　early　diagnosis　of　HER2-dependent　breast　cancers　in　clinical　diagnosis.