The extraction of calcium resources from high-calcium fly ash and the preparation of nano-CaCO3 were conducted in order to realize high value utilization of high-calcium fly ash. NH4Cl was used as an extractant to extract calcium from the high-calcium fly ash. About 50% ammonia in the extraction solution was separated and used to prepare NH4HCO3 solution, and then nano-CaCO3 was prepared with a microchannel reactor by separated nucleation/crystallization technology using NH4HCO3 and extraction solutions as raw materials. The obtained nano-CaCO3 was modified by sodium stearate and subsequently mixed with polypropylene (PP) to prepare CaCO3/PP composites. FT-IR, XRD and SEM characterizations were conducted to analyse the morphology and structure of the prepared nano-CaCO3 and CaCO3/PP composites. The results showed that CaCO3 with high dispersion, uniform size and an average particle size of 45.0 nm was prepared by using 0.5 mol/L CaCl2 solution and 15% ammonium citrate, nucleating at 30 ℃ and crystallizing for 5 min. The sodium stearate modified CaCO3 nanoparticles were uniformly dispersed in the CaCO3/PP composites, which improved the mechanical properties of the composites. The tensile strength, elastic modulus and breaking elongation of CaCO3/PP composites were about 35.6 MPa, 999.8 MPa and 10.26% when 6 wt% nano-CaCO3 was added in PP, which were about 14.5%, 3.2% and 21.0% higher than those of pristine PP (31.1 MPa、968.7 MPa和8.48%), respectively. The modified nano-CaCO3 particles have a strong interaction with PP, and the modified nano-CaCO3 particles are easy to slide on the molecular chain of PP and can absorb the fracture energy during stretching, resulting in the improved toughness of the composite.