<p>This study investigated the effect of calcium oxide (CaO) on the direct reduction process of Hongge vanadium titanomagnetite pellet (HVTMP) using hydrogen-rich gases. The findings indicated that CaO significantly adversely impacted the reduction degree, primarily due to the formation of difficult-to-reduce compound CaFe₂O₄. Increasing the CaO content in the Hongge vanadium titanomagnetite resulted in a decreased metallization degree. An analysis of the effect of CaO on the morphological transition and growth of metallic iron revealed that while the addition of CaO decreased the reduction swelling and significantly enhanced the compressive strength of reduced HVTMP up to an addition of 1%, further increase in CaO led to an increase in the reduction swelling. A conceptual diagram was developed to illustrate the relationship between CaO addition, reduction swelling, and the growth of metallic iron whiskers. 1% CaO addition could be considered an effective means of decreasing the reduction swelling of HVTMP. This study provides insights for optimizing the gas-based reduction of HVTMP and offers a theoretical and technical foundation for the utilization of HVTM.</p> Graphical Abstract <p></p>