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Utilizing super-concentrate for direct hydrogen reduction in pelletizing which decrease energy consumption and improve smelting quality are critical steps towards achieving the dual carbon goal. The reduction swelling behavior and its corresponding mechanism of ultra-high-grade pellets were studied from multi-dimensions combing visualization detection, micromorphological analysis, and first-principles molecular dynamics simulation in present work. The results showed that for the reduction no matter in CO or H2atmosphere, the ultra-high grade pellets expand abnormally. Whereas the increase of SiO2content and the selection of H2atmosphere can effectively inhibited the reduction swelling behavior of pellets, while Al2O3content had only a slight effect. The cause of catastrophic expansion was related to the various formation of new-born iron that it exhibited angular during CO reduction while became flatter with SiO2addition or by H2reduction. The simulation results were validated the accuracy since the structure and energy changes during iron oxide reduction were corresponding to the experimental observations. C and Fe mainly formed chemical bonds while H and Fe primary produced physical adsorption during FexO reduction. Combing the comprehensive effect of adsorption energy, electrostatic force, and energy change, it was tended to create iron whiskers by CO reduction contrary to H2reduction. It was conductive to generate iron whisker after Si doped in FexO. However, the decrease of pellets RSI with SiO2addition in H2reduction was caused by the increase of pellet porosity, which provided space for the growth of iron whisker. With the increase of Si doping amount in CO reduction, iron whiskers gradually changed from thin to chunky, resulting in more uniform growth of iron whiskers and uniform stress, which brought about a decrease in pellet reduction expansion rate contrarily.