Abstract: The Tarim fracture-cave carbonate formation has developed karst caves, which are prone to overflow and leakage during drilling, making pressure control difficult and posing great challenges to drilling and well control safety. It is urgent to study the gas-liquid displacement law and pressure change law when encountering a constant volume body during drilling. During the gas-liquid displacement process of the constant volume body, drilling fluid leakage and gas enter the wellbore. The two-phase flow phase-field method can effectively simulate the characteristics of the gas-liquid displacement interface and pressure changes in the constant volume body. Based on the two-phase flow phase-field method, the gas-liquid displacement law when drilling into a constant volume body was studied. Compared with on-site data, the reliability of this method in simulating the gas-liquid displacement law when drilling into a constant volume body was verified. The pressure variation characteristics and gas-liquid displacement laws were simulated and analyzed under different volumes of constant volume body, drilling depths, and leakage pressure differences. The results indicate that as the volume of the constant volume body increases, the amount of gas entering the wellbore increases when the same volume fraction of drilling fluid is lost. A greater amount of gas enters the wellbore when there is a deeper constant volume body. An increase in initial leakage pressure difference can lead to a significant decrease in the amount of gas entering the wellbore. When encountering a constant volume body during drilling under high initial leakage pressure difference conditions, it is prone to backflow leakage. The research results provide a theoretical basis for the development of treatment plans when encountering constant volume body during on-site drilling.