Abstract: The gas kick represents a major risk in deepwater oil and gas exploration. Understanding the dynamics of gas kick evolution and the associated pressure response characteristics is critical for effective well control. In this paper, we introduce a transient wellbore multiphase flow model specifically developed to simulate gas kick in deepwater dual-gradient drilling, incorporating a downhole separator. The model accounts for the variable mass flow within the annulus and heat exchange between the annular fluid and the formation. Using this model, we analyzed the multiphase flow and thermodynamic behavior during the gas kick. Simulation results reveal a progressive increase in bottom-hole temperature, underscoring its potential as a key indicator for gas kick early detection. Additionally, variable gradient parameters affect not only the annular equivalent circulating density (ECD) profile but also the evolution of the gas kick. The inclusion of a downhole separator alters the annular ECD profile, creating a “broken line” shape, which enhances adaptability to the multi-pressure systems typically encountered in deepwater formation. By adjusting factors such as hollow sphere concentration, separator position, and separation efficiency, the annular ECD profile can be effectively customized. This study provides important theoretical insights and practical applications for utilizing dual-gradient drilling technology to address challenges in deepwater formation drilling.