Lysine demethylase 5B (KDM5B) is a member of the Jumonji AT-rich interactive domain 1 family. Its main function is to demethylate di/trimethyl histone H3 lysine 4 and it plays a crucial role in the occurrence and development of cancer. In this study, we performed structure-based optimization of KDM5B inhibitors based on our previous work and the most active compound we synthesized was 11ad. Molecular modeling studies and thermal shift assays revealed that 11ad specifically targets KDM5B at the molecular and cellular levels. Crucially, 11ad demonstrated good pharmacokinetic properties and anti-prostate cancer activity in a xenograft model. Furthermore, unexpectedly, the specificity of 11ad for prostate cancer was found to be related to its inhibition of the phosphoinositide 3-kinase/AKT pathway. This is the first report of a KDM5B inhibitor affecting this pathway. Taken together, our findings indicate that 11ad is a novel KDM5B inhibitor that may serve as a lead compound for the development of treatments for prostate cancer.