CO2 injection after water flooding in low permeability reservoirs can significantly improve oil recovery, but the effects of water cut conditions and key parameters on miscible ability, oil displacement and storage efficiency need to be clarified. In this paper, the indoor slim-tube experiment of CO2 continuous flooding after water flooding is carried out to determine the influence of water phase on miscible ability. The water-CO2 alternate flooding experiment of composite long core with nuclear magnetic resonance (NMR) test has clarified the optimal displacement mode and the influence of displacement pressure, and summarized the influence of slug size and slug ratio on the efficiency of oil recovery and storage efficiency. The results show that when the original miscible pressure is low (less than 25 MPa), the miscible pressure will increase by more than 30% under high water cut conditions. When the original miscible pressure is high (greater than 30 MPa), the water cut has little effect on the miscible pressure. After water flooding, the effect of gas injection in the reservoir has a ‘platform period’, and the CO2 water-alternative-gas method achieves higher recovery factor. Increasing the pressure can overcome the water shield barrier effect to a certain extent, and the recommended pressure level is 1.1 MMP. Considering the full process of displacement, the recovery is the highest when the water–gas slug ratio is 1: 1, and the storage efficiency is the highest when the slug ratio is 2: 1. When the slug size is 0.1 PV, the oil displacement and storage effects are optimal. The research results provide reference for gas injection development of low permeability reservoirs after water flooding.