Nano-CaCO3 was prepared from phosphogyp-sum sum (PG) through reactive crystallization in the gas (CO2)-liquid (NH3-H2O) solid (PG) three-phase system. The effects of temperature, CO2 flow rate, and reaction time on the CaCO3 particle size distribution (PSD) were investigated. The results show that PG-derived nano-CaCO3 with an average particle diameter of 86-104 nm can be obtained under a 251-138 mL/min CO2 flow rate at a corresponding temperature of 30-40 degrees C. Research indicates that the CaCO3 particle size is subjected to the mutual influences of temperature, CO2 flow rate, and reaction time. A relatively low CO2 flow rate with increasing temperature and a strictly controlled reaction time are advantageous to the formation of nanosized CaCO3 grains. Additionally, PG-derived nano-CaO maintains a CO2 sorption capacity of 0.27 g of CO2/g of CaO after 10 calcination carbonation cycles, which is the same as for commercial nano-CaO.