Based on the strategic design of intelligent nano-sized oil-displacement agent with three properties of ultra-small size， super lipophilicity and hydrophobicity，and coalescence of dispersed oil droplet，innovative idea of nano-sized water and nano-sized oil was put forward to overcome the technical bottleneck of“no injection，no production”of low-permeability reservoir. By means of modifying nano-sized particle with functional groups，the first generation of nano-sized oil-displacement agent， iNanoW1.0，was developed. Normal water could be transformed into“nano-sized water”in presence of iNanoW1.0 by weakening or destroying the association of hydrogen bonds between water molecules. Through the molecular dynamic simulation and capillary analysis，the effect of adding medium to water on the hydrogen bond by weakening or breaking between water molecules was analyzed. By conducting low field nuclear magnetic resonance displacement experiment and core displacement experiment，the injection performance and displacement effect of iNanoW1.0 and normal water were compared. The pilot test with iNanoW1.0 as oil-displacement agent was carried out in Changqing oilfield of ultra-low permeability reservoir. The results showed that iNanoW1.0 could transform normal water into nano-sized water by destroying the dynamic network structure of water molecule. Compared with normal water，iNanoW1.0 possessed better injection performance and displacement effect. With lower starting pressure gradient， iNanoW1.0 could expand the swept volume in extra-low or ultra-low permeability reservoir which could not be swept by normal water. The field pilot test in Changqing oilfield of ultra-low permeability reservoir had the characteristics of increasing fluid， increasing oil and lowering the rate of decline. The effect of enhanced oil recovery by waterflooding was observed for the first time in Changqing oilfield of ultra-low permeability reservoir. The theory of waterflooding in low-permeability reservoir was developed.