In order to deal with the overflow accident caused by gravity displacement, it is necessary to understand the conditions of gravity displacement, analyze the gravity displacement window to judge whether gravity displacement occurs in the down-hole, and adopt the corresponding treatment method. The critical point experiment of gas-liquid gravity displacement was carried out by the self-developed well-formation gravity displacement experimental device. Based on the theory and experimental analysis, the formation gravity displacement window was quantitatively calculated, and the critical point pressure curve of the gravity displacement window was fitted. The error between the fitting formula and the measured value is less than 10%. Under this experimental conditions, when the drilling fluid density is 0.998 g/cm³, the gas-liquid gravity displacement window is -5 800 ~ 10.6 Pa, and the liquid-liquid formation gravity displacement window is -1 192 ~ 16.6 Pa. The results show that the gravity density difference between gas and fluid is linear, and the actual gravity displacement window is much higher so that the influence of gravity displacement window should be considered during drilling engineering. In conclusion, the prevention method of gravity displacement and the corresponding treatment method of gravity displacement overflow are put forward.