AbstractA series of indirubin derivatives hydrochloride were obtained using a convenient and mild method from indirubin. The newly synthesized compounds and their derivatives were characterized by 1H NMR, 13C NMR, and MS. Furthermore, we tested their IDO1 enzyme inhibitory activity and anti‐proliferative activities in K562 tumor cells. The experimental results revealed that some of the compounds showed potential anti‐enzyme and anti‐tumor activity. Among of them, compound 4b exhibited certain inhibitory activity on IDO1 (IC50: 29.52 μmol/L), and compound 4a displayed activity similar to indirubin in K562 tumor cells (IC50: 24.96 μmol/L). This type of indirubin derivative is expected to have the potential to act as an immunologically active anti‐tumor drug, indicating that further research will be worthwhile.