Karst caves are the main storage space of fractured-cavity reservoirs. Due to karstification, the development of the caves is complicated. Because of the difference in density between oil and water, the residual oil is enriched in the upper part of single wells during water-flooding. In order to study the oil displacement effect of nitrogen foam flooding under different injection parameters after water flooding, a visual cave model of a fractured-cavity reservoir was designed and fabricated to carry out an experimental study of injection parameters of nitrogen foam flooding and visualize the oil displacement effect. The experimental results show that in terms of the cave reservoir body, there exist optimal nitrogen foam slug sizes, which vary with the characteristics of the cave and remaining oil distribution. Compared with nitrogen foam flooding after water flooding, the nitrogen foam flooding after bottom water drive could shorten the high water cut period, reduce both the water injection amount and the cost of water injection. The distribution of remaining oil after nitrogen foam flooding by a low-position well or a high-position well was different. Besides, the nitrogen foam and the bottom water could generate a synergistic effect, which changed the oil and water distribution in part of the karst cave after water flooding. The main EOR mechanism of nitrogen foam is gravity differentiation and reducing interfacial tension between oil and water. We defined a three-phase gravity criterion of foam, oil and water to describe the synergistic effect of foam and bottom water. The reduction of interfacial tension resulted in redistribution of the flow field. The results of laboratory experiments can provide reference for the development of the remaining oil in a cave body reservoir.