The fluid inclusions in mantle rocks and melt indicated that a large amount of CO2 fluid exists in the deep earth, which is of great significance for understanding the deep carbon cycle and the composition of mantle. However, it was also suggested that carbonate minerals were likely to be the main host of mantle carbon. At the same time, the distribution and behavior of carbon in the mantle still remain a puzzle. In this paper, the adsorption behavior and occurrence characteristics of supercritical CO2 in magnesite (MgCO3) pores were studied by the Grand Canonical Monte Carlo method (GCMC) under the different conditions of CO2 pressures (0–100 MPa), temperatures (350–1500 K) and the pore sizes (7.5–30 ?). The simulated results showed that the adsorption of CO2 in magnesite was a physical adsorption, which was mainly controlled by the intermolecular force. The gas adsorption became more stable when the adsorption site shifted from the high energy site to the low energy site with increasing pressure (P) and decreasing temperature (T) and pore size. At the same time, the variations of excess adsorption amounts of CO2 in the pores of magnesite (Nexcess) under the different conditions were quan-titatively calculated. It was found that the Nexcess decreased with increasing T, but increased with increasing P and pore size. The results favor understanding the CO2 migration, seismic precursor observations, and heat transfer process in the deep earth.