Abstract: The Tarim fracture cave type carbonate rock 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 cave during drilling. During the process of constant volume gas-liquid displacement, drilling fluid leakage and gas entering the wellbore are accompanied. The two-phase flow phase-field method can effectively simulate the characteristics of gas-liquid displacement interface and pressure changes in the constant volume cave. Based on the two-phase flow phase-field method, the gas-liquid displacement law when drilling into a constant volume cave 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 cave has been verified. The pressure variation characteristics and gas-liquid displacement laws were simulated and analyzed under different constant volume caves, drilling depths, and leakage pressure differentials. The results indicate that as the volume of the constant volume cave increases, the amount of gas entering the wellbore increases when the same volume fraction of drilling fluid is lost. When the same volume fraction of drilling fluid is lost, a greater amount of gas invasions into the wellbore by the deep constant volume cave. An increase in initial leakage pressure difference can lead to a significant decrease in the amount of gas entering the wellbore when the same volume fraction of drilling fluid is lost. When encountering a constant volume cave 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 cave during on-site drilling.