This paper proposes an estimating bottom-hole pressure under abnormal conditions and judge abnormal working conditions method for the special conditions where gas invasion may occur downhole in the process of managed pressure drilling. Firstly, based on the simplified drift flux model, the gas-liquid two-phase flow model of managed pressure drilling is established by adding the gas expansion term to the traditional hydraulic model. Then, considering the downhole annular friction integral and bottom-hole gas influx as unknown parameters, and the bottom-hole flow as unknown state. An adaptive observer that can jointly estimate unknown parameters and unknown state is designed, and the bottom-hole pressure is estimated based on the obsever. Simulation experiments verify the effectiveness of the proposed control method. The simulation results show that the gas-liquid two-phase flow model can simulate the change of bottom-hole pressure when gas kick occurs downhole, and the observer can accurately track the changes of bottom-hole flow and pressure in real time.