In this work, the volume swelling and carbon deposition behavior of pellets under shaft furnace smelting conditions were studied. The research process on swelling reduction utilized an online detection imaging method to determine the volume changes of pellets. The results suggest that an increase in temperature and a higher proportion of carbon monoxide in the reducing gas contribute to an escalation in the swelling rate during pellet reduction. Carbon deposition induced by carbon monoxide primarily occurs at 750 °C, and the powdery carbon deposition products on the surface of the pellets change over time, transitioning from carbon to a mixture of carbon and iron carbide (Fe3C) to a mixture of carbon, Fe3C, and metallic iron. This study elucidated the behavior of pellets in the low-temperature reduction section during the shaft furnace smelting process, offering a technical foundation for shaft furnace smelting technology. Display omitted •The volume change of the pellet during the hydrogen rich reduction process was observed using a real-time camera system.•Investigated the reasons for variations in pellet reduction swelling index under different gas mixtures.•Investigated the mechanism of carbon deposition on the surface of pellets with hydrogen-rich gases.•Investigated the evolution mechanism of carbon deposition products at different stages.