Nano-CaCO3 was prepared by liquid-phase precipitation using a micro-sieve reactor with Ca(NO3)2·4H2O as calcium source continuous phase and (NH4)2CO3 as carbon source dispersed phase. The effects of the preparation conditions such as flow rate, concentration and residence time of the dispersed and continuous phases on the particle size, yield and morphology of calcium carbonate were investigated by XRD and TEM. The results show that the calcite nano-CaCO3 prepared by liquid-phase precipitation in micro-sieve reactor has an average particle size from 45 nm to 92 nm and a yield of more than 80%. The suitable preparation conditions are:continuous phase feed flow FC=150 mL/min, dispersed phase feed flow FD=150 mL/min, continuous phase concentration ［Ca2+］=0.05 mol/L, dispersed phase concentration ［CO2-3］=0.2 mol/L and residence time τ=5 s. The unique pore structure of the micro-sieve reactor can evenly disperse the liquid-phase system, thus avoiding the clogging problem occuring in liquid-phase precipitation reactions in general micro reactors. The molecular diffusion and mixing mode of the micro-sieve reactor could homogeneously enhance the perturbation of the liquid phase system and increase the supersaturation of CaCO3, leading to smaller particle size with a very narrow distribution. Meanwhile, the shape and size of the samples can be flexibly adjusted by changing the preparation conditions.