JAKs inhibitors were widely applied in the treatment of immunodeficiency diseases, inflammation and cancers. We designed and synthesized a novel series of 4-aminopyrazole derivatives, which showed inhibitory potency against various JAKs. The in vitro protein kinase inhibition experiment indicated that compounds 17k, 17l, 17m and 17n could inhibit JAKs effectively. Among them, compound 17m possessed the highest protein kinase inhibitory rates (%) at 10μM, which were 97, 96 and 100 to JAK1, JAK2 and JAK3, respectively. Further evaluation revealed that the IC50 values of 17m against JAK1, JAK2 and JAK3 were 0.67μM, 0.098μM and 0.039μM, respectively. Moreover, western blotting results showed compound 17m could inhibit the phosphorylation of JAK2 in Hela cells effectively. The data supports the further investigation of these compounds as novel JAKs inhibitors.