KDM5B (also known as PLU-1 and JARID1B) is 2-oxoglutarate and Fe dependent oxygenase that acts as a histone H3K4 demethylase, which is a key participant in inhibiting the expression of tumor suppressors as a drug target. Here, we present the discovery of pyrazole derivatives compound 5 by structure-based virtual screening and biochemical screening with IC of 9.320 μM against KDM5B, and its subsequent optimization to give 1-(4-methoxyphenyl)-N-(2-methyl-2-morpholinopropyl)-3-phenyl-1H-pyrazole-4-carboxamide (27 ab), a potent KDM5B inhibitor with IC of 0.0244 μM. In MKN45 cells, compound 27 ab can bind and stabilize KDM5B and induce the accumulation of H3K4me2/3, bona fide substrates of KDM5B, while keep the amount of H3K4me1, H3K9me2/3 and H3K27me2 without change. Further biological study also indicated that compound 27 ab is a potent cellular active KDM5B inhibitor that can inhibit MKN45 cell proliferation, wound healing and migration. In sum, our finding gives a novel structure for the discovery of KDM5B inhibitor and targeting KDM5B may be a new therapeutic strategy for gastric cancer treatment.