Inhibiting PARP-1/2 offered an important arsenal for cancer treatments via interfering with DNA repair of cancer cells. Novel PARP-1/2 inhibitors were designed by capitalizing on methyl- or ethyl-substituted piperizine ring to capture the characteristics of adenine-ribose binding site (AD site), and their unique binding features were revealed by the cocrystal structures of compounds 4 and 6 in PARP-1. The investigation on structure-activity relationship resulted in compounds 24 and 32 with high enzymatic potency, binding selectivity, and significantly longer residence time for PARP-1 over PARP-2 (compound 24, PARP-1: IC50 = 0.51 nM, PARP-2: IC50 = 23.11 nM; compound 32, PARP-1: IC50 = 1.31 nM, PARP-2: IC50 = 15.63 nM). Furthermore, compound 24 was determined to be an attractive candidate molecule, which possessed an acceptable pharmacokinetic profile and produced remarkable antitumor activity in both breast cancer xenograft model and glioblastoma orthotopic model in mice, either alone or in combination treatment.