p300 is an important histone acetyltransferase in epigenetics, and its overexpression is closely related to various diseases such as cancers. C646 is one of the most representative p300 inhibitors and used in various studies of p300. However, its intrinsic drawbacks such as containing potentially toxic groups prevent it from further development. In order to find potent p300 inhibitors with good drug-like properties, C646 was chosen as the lead compound and a series of new p300 inhibitors were designed based on the principle of bioisosterism and reasonable scaffold hopping, and the structure-activity relationship was systematically explored. Ten of them were found to show comparable inhibitory activity as C646. The most potent compound, 1r (IC = 0.16 μM), showed better p300 inhibitory activity than C646 with improved drug-like properties. Western blotting experiment confirmed that 1r could reduce the level of H3K27 acetylation more significantly than C646. Further cellular assay indicated that it could inhibit the proliferation of human breast ductal carcinoma cell T47D and human breast cancer cell MCF7 with the IC values of 5.08 μM and 22.54 μM, respectively. Docking study of 1r with p300 protein showed the possible reasons for its higher inhibition activity. Thus, compound 1r might be with potential for development as a novel epigenetic agent targeting p300.