As our research focuses on anticancer drugs, a series of novel derivatives of flexicaulin A (FA), an ent-kaurene diterpene, condensed with an aromatic ring were synthesized, and their antiproliferative activities against four human cancer cell lines (TE-1, EC109, MCF-7, and MGC-803) were evaluated. The activities of most of the new compounds were better than those of FA. Compound 2y exhibited the best activity with an IC50 value reaching 0.13 μM against oesophageal cancer cells (EC109 cells). The IC50 values for 2y in normal cells (GES-1 cells and HUVECs) were 0.52 μM and 0.49 μM, respectively. Subsequent mechanistic investigations found that compound 2y can inhibit the proliferation of cancer cells and cell cloning. In addition, 2y could reduce the mitochondrial membrane potential, increase the apoptosis rate, and increase the ROS level in EC109 cells. Moreover, 2y can upregulate the expression of ROS/JNK pathway-related proteins (p-ASK1, p-MKK4, p-JNK, and p-Cjun (ser63)) and pro-apoptotic proteins (Bax, Bad, and Bim). In vivo experiments showed that 2y can inhibit tumour growth in nude mice. The mechanism involves an increase in protein expression in the ROS pathway, leading to changes in apoptosis-related proteins. In addition, compound 2y shows low toxicity. These results indicate that compound 2y holds promising potential as an antiproliferative agent.