At present, the mechanism of nano-displacement agent to improve the recovery efficiency of ultra-low permeability reservoir has not been studied and researched systematically in China. Because of the super ultra-low permeability of Changqing Oilfield, the application of nano-emulsion agent in such reservoir is still in initial stage. In order to improve the displacement efficiency, nano-emulsion flushing technology was developed in Jiyuan reservoir, Changqing Oilfield. The guided test of two wells was carried out in Block Z218. In the laboratory, the fractions, the mechanism of action, the interfacial tension and the relationship between concentration and particle size were analysed, the temperature and salt resistance, adsorption loss and oil displacement were evaluated, the initial field application result was summarized. The current pressure and water content are stable and the amount of oil is rising. It showed that nano-emulsion flushing technology has a good application prospect in ultra-low permeability reservoir. For weak hydrophilic reservoirs, nano-emulsion has good injectivity, the hydrophilic pore throat channel has been removed, the efficiency of oil displacement has been improved and the damage to stratum has been reduced. Eventually, a long-term oil and gas recovery mechanism could have been formed and the ultimate reservoir recovery efficiency could have been enhanced. For Changqing Oilfield, the only mature technology to improve ultra-low permeability-compact reservoirs is refracturing, the significance of the new nano-displacement technology is more economical and convenient than refracturing.