Thermal decomposition is the primary step to utilizing magnesite resources. Traditionally, it has been done in the shaft or rotary-type kilns using lumpy bulk raw materials, leaving a substantial amount of small or millimeter-sized particle materials unusable. In this work, the decomposition of millimeter-scale magnesite particles using high-temperature gas-solid fluidized beds is proposed. The thermal decomposition behavior of magnesite particles with four different sizes in the range of 0.3~3 mm at different temperatures using a laboratory fluidized bed reactor of 30 mm in diameter combined with an online mass spectrometry analyzer is reported in this work. The results showed that the thermal decomposition rate of magnesite particles accelerated with the increase in bed temperature and the decrease in particle size. As the decomposition progresses, three characteristic stages can be observed. At the initial stage of decomposition corresponding to conversions of less than about 0.1, the reaction was controlled by the interfacial chemical reaction kinetics, and the apparent activation energy decreased with increasing particle size. In the middle stage of the reaction (the conversion was 0.1~0.9), the decomposition reaction obeyed the shrinking core model, and the activation energy of the decomposition reaction remained almost unchanged with the conversion for the particle of 0.46 mm in diameter but increased with the conversion for other large-sized particles. In the later stage of the thermal decomposition reaction (after the conversion was greater than 0.9), the conversion varied slowly with time, and the reaction was affected significantly by heat transfer and gas diffusion. For each of the decomposition reaction stages, the reaction mechanisms and kinetic parameters were determined based on the experimental data. This study discussed the effects of bed temperature and particle size on the thermal decomposition of magnesite particles for a better understanding of the thermal decomposition behavior of millimeter-sized magnesite particles in fluidized beds. The study provided essential data support for the development of new products for the preparation of magnesite particles in fluidized beds, serving as a critical reference for the preparation of millimeter-sized high-density dead burned magnesia at high-temperature in fluidized beds.