Abstract: Using ¹⁷O nuclear magnetic resonance (¹⁷O-NMR) technology, this paper studies the feasibility of preparing nano-oil-displacing agent with iNanoW1.0 nano particles and on-site water from the Changqing Jiyuan oilfield. The experimental results show that iNanoW1.0 nano particles can weaken the hydrogen-bonding interaction between on-site water molecules with high salinity. For the on-site water of the Changqing Jiyuan oilfield, the nano-oil-displacing agent with the mass fraction of 0.1% is the optimal working concentration. The low-field nuclear magnetic resonance (LF-NMR) core displacement experiment is used to study the displacement effect of nano-oil-displacing agent on low-ultra-low permeability cores. The experimental results show that the nano-oil-displacing agent prepared with on-site water can effectively reduce the starting pressure of fluid in the low-ultra-low permeability core, especially for the small pore part of the core, swept volume enlargement is obvious, which can realize the enhanced oil recovery (EOR) of 10.29% -11.44%.