Dietz model is a common method to determine whether the gas-liquid interface is stable in inclined reservoirs, but this model does not consider the influence of elevation difference between injection and production wells, well spacing, reservoir rhythm and pressure drop funnel of liquid production wells on the stability of gas-liquid interface. The maximum angle of gas-liquid interface is quantitatively expressed by introducing the elevation difference between injection and production wells and the plane well spacing. According to the rhythm characteristics and injection-production ratio, a new method for determining the stability velocity of gravity-assisted flooding of gas injection is further given, and the stability conditions of secondary gas top of LBIV oil group are analyzed. The results show that this method can be used for the dynamic analysis of enhanced oil recovery by gas injection gravity-assisted flooding after high-angle water flooding.