Nano calcium carbonate with particle size ≤100 nm is easy to agglomerate and has poor dispersion.When it is used in sealant，there is some disadvantages of poor extrusion performance and high modulus.To overcome these shortcomings，the effect of key parameters such as the activity of lime，the particle size of Ca（OH）₂ in lime slurry，different carbonation rate and slurry feeding amount on the synthesis of cubic calcium carbonate were studied.Submicron calcium carbonate was prepared under the experimental conditions：the activity of lime was 330~350 mL，the average particle size of Ca（OH）₂ slurry was smaller than 2 μm，carbonization rate was 30%~65% and the amount of slurry supplement was 20%~50% of the original slurry.The synthesized calcium carbonate was characterized by scanning electron microscope and laser particle size analyzer.It was found that the prepared calcium carbonate was cubic，the particle size distribution was 100~400 nm，and the specific surface area was 10~15 m²/g.When it was applied to MS sealant，it had good extrudability，high strength and elongation，low modulus and the best comprehensive performance.It met the standard of low modulus sealant in GB 14683—2017 silicone and modified silicone building sealant.