The approach of using ferroptosis to treat cancer has garnered attention due to its promising potential. However, the effectiveness of this therapy is often limited by the inherent redox system in cancer cells and the presence of ferritin as an iron ion storage molecule. To address this issue, we have designed a polymeric iron oxide nanocomplex loaded with sulfasalazine as a ferritinophagy-assisted ferroptosis inducing agent. The nanocomplex is based on a pH-responsive drug releasing platform that enables improved ferroptosis anti-cancer therapy. The nanocomplex was synthesized using polymerized phenylboronic acid decorated with iron oxide and further loaded with sulfasalazine by interacting with polymerized phenylboronic acid. Upon entering cancer cells, the nanocomplex releases sulfasalazine at the lysosomal acidic pH, which causes the complex to degrade into the labile iron ion (Fe2+). This process inhibits the production of GSH and reproduces the labile iron ion by degrading ferritin. As a result, an excess iron ion pool is formed, and the facilitated Fenton reaction induces an improved ferroptosis anti-cancer effect. Moreover, our research has demonstrated that the nanocomplex effectively regresses tumors, thereby representing significant inhibition of tumor growth using in vivo models. We believe that this ferritinophagy-assisted ferroptosis strategy using the nanocomplex provides a promising solution for iron-based anti-cancer therapy.