A series of novel coumarin derivatives containing 4,5-dihydropyrazole moiety as potential telomerase inhibitors were synthesized. The bioassay tests show that compound 3d exhibited potentially high activity against human gastric cancer cell SGC-7901 with IC(50) value of 2.69 ± 0.60 μg/mL. All title compounds were assayed for telomerase inhibition by a modified TRAP assay, the results show that compounds 3d and 3f can strongly inhibit telomerase with IC(50) values of 2.0 ± 0.07 and 1.8 ± 0.35 μM, respectively. Docking simulation was performed to position compound 3d into the telomerase (3DU6) active site to determine the probable binding model.