In this work, a numerical simulation model of an industrial-scale magnesite flash calciner (MFC) was established based on computational fluid dynamics (CFD) method. The discrete phase model (DPM) was employed, and the results of kinetic analysis for magnesite decomposition were taken into consideration. Then, the influence of swirling gas inlet design on particle motion and decomposition in MFC was analyzed, thereby providing suggestions for production. The results indicate that incorporating a swirling design can attenuate the particle deposition at the feeding port and increase particle residence time in furnace. However, the conversion degree of magnesite is decreased. This phenomenon can be attributed to the particle accumulation near the wall, resulting in a localized lower gas temperature and subsequently leading to a gas-solid heat transfer degradation. By elevating the gas temperature and reducing its flow rate, the enhancement of magnesite decomposition and reduction in energy consumption are achieved. The gas-solid water equivalent ratio, determined by heat of reaction and sensible heat of flue gas, needs to be higher than 1.8 to maintain a mass fraction of MgO above 0.9. Display omitted •Effect of swirling gas inlet on magnesite flash calciner performance is studied.•The particle motion and reaction behavior in furnace are analyzed using CFD method.•The particle motion is improved while the mass fraction of MgO is reduced.•The gas-solid equivalent ratio is introduced and a practical range is obtained.