Abstract: Artificial gas cap flooding can be used to exploit the locally enriched remaining oil in high-angle structural oil reservoirs, and the formation boundary of gas cap is an important factor influencing the feasibility of artificial gas cap flooding. In order to define the theoretical formation boundary of artificial gas cap, this paper established a microscopic control model of gas bubble according to the mechanical characteristics of gas bubbles in porous medium. Then, sensitivity analysis was carried out on the main control factors of the model. Finally, the minimum gas column height needed for the formation of artificial gas cap during nitrogen displacing water and nitrogen displacing oil under the conditions of laboratory simulation and oil reservoirs was determined. It is indicated that the minimum gas column height needed for the formation of artificial gas cap increases with the increase of interfacial tension, oil reservoir pressure and reverse included angle between capillary force and mass force, and decreases with the increase of permeability, density difference and stratigraphic dip. The key to the formation of artificial gas cap is to reduce the interfacial tension. Under the condition of laboratory experiment, the minimum gas column height for nitrogen displacing oil is 65.4 cm and that for nitrogen displaceing water is 86.9 cm. And under the condition of oil reservoir, they are 42.9 cm and 93.5 cm, respectively, and the critical gas column height for the initiation of gas bubble is much larger than the size of long core. The research results provide the technical support and theoretical guidance for the development and production of similar oil and gas fields.