Calcination of MgCO 3 is an important industrial reaction, but it causes significant and unfavorable CO 2 production. Calcination in a reducing green hydrogen atmosphere can substantially reduce CO 2 release and produce high value-added products such as CO or hydrocarbons, but the mechanism is still unclear. Here, the in situ transformation process of MgCO 3 interacting with hydrogen and the specific formation mechanism of the high value-added products are thoroughly investigated based on reaction thermodynamic, ab initio molecular dynamics (AIMD) simulations, and density functional theory (DFT) calculations. The reaction thermodynamic parameters of MgCO 3 coupled with hydrogen to produce CO or methane are calculated, revealing that increasing and decreasing the thermal reductive decomposition temperature favors the production of CO and methane, respectively. Kinetically, the energy barriers of each possible production pathway for the dominant products CO and methane are further calculated in conjunction with the AIMD simulation results of the transformation process. The results suggest that CO is produced via the MgO catalytic-carboxyl pathway (CO 2 *→ COOH* trans → COOH* cis → CO*→ CO), which is autocatalyzed by MgO derived from the thermal reductive decomposition of MgCO 3 . For the mechanism of methane formation, it prefers to be produced by the stepwise interaction of carbonates in the MgCO 3 laminates with hydrogen adsorbed on their surfaces (direct conversion pathway: sur-O-CO → sur-O-HCO → sur-O-HCOH → sur-O-HC → sur-O-CH 2 → sur-O-CH 3 → sur-O + CH 4 *).