Highly dispersed nano calcium carbonate was prepared using limestone as raw material by low-temperature carbonization method. The nano calcium carbonate samples were characterized by XRD, SEM, TEM, etc. The effects of slurry ratio, carbonization temperature, CO2 aeration rate, aging time, and crystal control agent on the particle size and dispersion degree of nano calcium carbonate were investigated. The process conditions for preparing nano calcium carbonate were obtained as follows: slurry ratio of 5%, carbonization temperature of 15 ℃, CO2 aeration rate of 200 mL/min, aging time of 2 h, and ZnCl2 as the crystal control agent. Under these conditions, cubic nano calcium carbonate with a grain size of 25~35 nm and uniform distribution was prepared. The growth mechanism of nano calcium carbonate was analyzed by Materials Studio. The results show that the (104) and (116) crystal planes of calcium carbonate were thermodynamically stable and would eventually become tenacious exposed crystal planes, which promots the generation of cubic nano calcium carbonate.