Abstract: During the deepwater riserless drilling process, the fluid movement law is unclear after a blowout occurs when shallow-layer gas is encountered. To address this issue, the Mixture multiphase model was adopted to simulate the flow process of the fluid after blowout. By considering the distribution characteristics of seawater temperature and pressure, a deepwater riserless drilling blowout model including the sea area and the drilling wellbore was established, and the accuracy of the model was ensured through comparison with VDROP−J blowout plume model for shallow water wells. Then, combined with the geological characteristics and physical properties of fluids in the South China Sea, the formation and evolution laws of plumes after blowouts were analyzed using the established model: after being ejected from the wellhead, the fluid would go through stages such as turbulent jet, suction and bending, and convective diffusion, eventually forming a plume that is narrow at the bottom and wide at the top. During the migration process, the fluid came into contact with seawater to form natural gas hydrates, which decomposed at a certain height due to changes in environmental pressure and temperature. Finally, based on the simulation results of multi-condition blowout plumes, a quantitative risk assessment model for blowout plumes was established through dimensionless number analysis. According to the hazard classification coefficient of blowout plumes, the risks of blowout plumes were divided into six levels, and the corresponding risk assessment charts were formulated. Based on the principle of safety barriers, prevention and control methods for deepwater riserless drilling blowouts were constructed from three aspects: prevention, mitigation, and control. Studies show that the established model can predict the movement range of blowout plumes, assess the safety risks of blowout plumes, provide a scientific basis for formulating response measures after blowout in deepwater riserless drilling, and help improve the safety and reliability of deepwater drilling operations.