Abstract: A series of phthalazine ketone compounds were synthesized and the structures were confirmed by ¹H NMR and HR-MS spectrum. All target compounds were obtained through 7 steps, including selective reduction, nitration, bromination, ring enlargement, reduction, Knoevenagel and acylated reaction. The compounds were evaluated for their immunosuppressive effects of T-cell proliferation and inhibitory activity of IMPDH type II in vitro, as well as their structure-activity relationship were assessed. Several compounds exhibited strong immunosuppressive properties, especially compounds 7f and 7h, with IC₅₀ values of 0.093 μmolSL^-1 and 0.14 μmolSL^-1 respectively, which were superior to mycophenolic acid. The information obtained from the studies may be useful for further research on the immunosuppressive agents.