The nanoparticles of calcium carbonate were synthesized from solid mixed salts to tackle the problems of difficult treatment of solid mixed salts and high preparation costs of nanoparticles，which were applied in nano-fluid flooding as the tertiary oil recovery technique to enhance oil recovery. Under the foam atmosphere system，calcium carbonate and magnesium carbonate mixed nanoparticles were synthesized from the solid mixed salts rich in Ca2+ and Mg2+ provided by the oil and gas mine water treatment plant in southwest Sichuan. The nanoparticles were dispersed in the polyacrylamide solution to form a nano-fluid flooding system，and the oil displacement effect was studied. The results show that the synthetic particles have an average particle size of 120.9 nm，a peak particle size of 111.7 nm，and a particle size distribution range of 28.87-638.78 nm. The particle surface adsorbs the side chain of alkyl sulfate，and the contact angle is 93.570°. When 1 g of synthetic nanoparticles are dispersed in the polyacrylamide solution，the nano-fluids can maintain stability for more than 24 h. In addition，displacement experiments show that when the nano-fluids are injected at a water cut of 80%，the nano-fluid flooding can form a stable displacement pressure difference of about 3 MPa. The oil recovery under the nano-fluid flooding increases to 88.66% from 46.67% under water flooding，with an increase of 41.99%，which indicates that the nano-fluid system has good oil displacement performance.