A peptoid compound, a manufacturing method of a peptoid compound, an oligomer, a pharmaceutical composition, use of the pharmaceutical composition in the preparation of a medicament for detecting or diagnosing a disease related to tyrosine kinase HER2, and a kit for identifying circulating tumor cells are provided, the peptoid compound includes: a cysteine (Cys) subunit, a butanediamine (Nlys) subunit, a 3,4-methylenedioxybenzylamine (Npip) subunit, a 3-aminopropanic acid (Nce) subunit and a 1-naphthylamine (Na) subunit, and both the peptoid compound and the oligomer have a strong ability to bind to HER2 protein on surfaces of circulating tumor cells (CTCs), and a technology of diagnosing breast cancer on the basis of the peptoid compound can realize non-invasive and label-free rapid diagnosis, in addition, the methods for synthesizing the peptoid compound and the oligomer are simple, the preparation efficiency is high, and the production cost is low.