The occurrence of formation loss and downhole overflow in the process of well control will increase the drilling risk. Once the control failure will lead to blowout accidents. In this paper, numerical simulation study is conducted during the overflow caused by leakage of formation underground. Combined with gas state equation and wellbore gas liquid two phase flow model, establish borehole overflow caused by leakage gas liquid two phase flow transient flow model. By making use of key parameters of an early stage accident well, analyse gas in the process of overflow rate, pressure and gas–liquid two phase flow distribution along the well depth in different time, It finds that the lower the initial liquid level height is, the more obvious the expansion phenomenon of the intrusive gas is. In the overflow process caused by formation loss induced by drillstring falling, the variation of gas content along well depth shows a three-stage distribution: there is a high gas content section at the bottom of wellbore, and the fluid composition is dominated by gas phase; Upward along the wellbore, the hydrostatic column pressure decreases and the gas expands, which is more obvious in the high gas content section. The middle part of wellbore is the mixed section, and the gas content of the mixed section decreases rapidly with the decrease of well depth. The gas content continues upward the wellbore and finally reaches zero, with pure liquid phase above. In the process of overflow caused by leakage, the bottom hole pressure increases first and then decreases with the increase of time. In the early stage of overflow, there is under pressure at the bottom of the well, and gas enters the formation at a high speed, resulting in an increase in the pressure difference at the bottom of the well; In the middle stage of overflow, when the wellbore is near equilibrium, the formation gas slowly invades and the bottom hole pressure difference slowly increases; In the later stage of overflow, after the wellhead returns, the static fluid column pressure decreases due to the return of drilling fluid, and the rate of decrease in bottom hole pressure gradually increases. The bottom hole pressure difference after the wellhead returns begins to decrease.