The specific fluorescent detection of α-methyltryptamine (AMT) presents a great challenge because similar amine groups and benzene rings exist in a variety of amines. Here, we show the precise modulation of the electron-withdrawing strength of the π-conjugate bridge in aldehyde-containing Schiff base-based fluorescent probes for ultratrace AMT discrimination. It is found that different electron-withdrawing groups -C6H4, -C6H2N2, and -C6H2Br2 as the π-conjugate bridge of the 2-dicyanomethylidene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran (TCF)-based probes can classify and identify organic amines with different amine nucleophilicities. Notably, the probe with -C6H2Br2 as the π-conjugate bridge, denoted as BrFS-TCF, which has the highest electrophilicity of the recognition site, shows a superior nM-level limit of detection (LOD) and an instant response time (<0.1 s) toward AMT. Especially, it shows an excellent selectivity facing the secondary amines, tertiary amines, aromatic amines, and even primary amines. The present strategy would provide a new pathway for chemical substances with similar structures and properties and especially for fighting against illegal drugs.