Ultra-deep carbonate reservoir drilling faces significant challenges in determining overflow formation pressure when overflow and lost circulation coexist. Traditional shut-in methods are compromised by lost circulation interference, making accurate pressure assessment difficult. This study introduces a novel method for inverting the overflow formation pressure coefficient using sudden hook load reductions and establishes a gas–liquid two-phase flow model for the wellbore under these challenging conditions. Data from six ultra-deep wells in the Tarim Basin reveal a strong correlation between gas influx and abrupt hook load drops. The analysis shows that high-pressure differentials force formation fluid to impact the drill string, triggering sudden hook load decreases. Additionally, the study quantifies the effects of bit size and well inclination on deriving formation pressure coefficients from hook load variations. Results indicate that the proposed method achieves an average error of only 1.98% compared to the actual pressure coefficient, outperforming the casing pressure-based approach, which has an error of 2.81%. Moreover, the developed multiphase flow model demonstrates that increased lost circulation rates reduce wellbore hydrostatic pressure, lower bottomhole flowing pressure, and intensify gas influx, thereby contributing to higher casing pressure. Study results show timely wellbore pressure control should be implemented when sudden hook load drop and abnormal flow occur during simultaneous overflow-loss circulation, providing theoretical support for ultra-deep carbonate drilling safety.