Due to the complex pore-throat structure and difficult development of tight reservoirs, conventional surfactants are easy to be adsorbed by rocks and cannot penetrate into the fine pore throat, which cannot meet the application requirements of enhanced oil recovery in harsh reservoirs. Therefore, the effect of improving spontaneous imbibition recovery is not obvious. In this paper, a nanoparticle microemulsion oil displacement agent system was studied and evaluated. The mechanism of oil displacement was revealed by interfacial tension test and wetting inversion test. The effect of EOR was verified by spontaneous imbibition test. The results show that the nano-emulsion system with 0.3% concentration has a particle size of 10 nm and an interfacial tension of 3.56 mN/m, which can reverse the wettability of oil-wet quartz surface (130°) to water-wet surface (11°), and has a good wetting inversion ability. The spontaneous imbibition recovery rate of 0.3% nano emulsion is 43.2%, which is about 2.5 times that of water. The spontaneous imbibition recovery rate is significantly improved. Nano-microemulsion can improve spontaneous imbibition recovery by reducing interfacial tension and changing wettability in the process of oil displacement in tight oil reservoirs. Among them, the change of wettability is the main mechanism to improve spontaneous imbibition recovery in tight oil reservoirs.