Abstract: With the increasing demand for energy and the decreasing availability of onshore oil and gas resources, offshore oil fields, desert oil fields, and marginal oil fields are being vigorously developed. In the process of oil and gas resource exploitation, oil and gas transportation technology is the key to cost savings, energy consumption reduction, and profit increase. Multiphase mixed transportation technology is widely applied due to its low investment cost, long-distance transportation, and high oil well production efficiency. As one of the core equipments in multiphase mixed transport, a multiphase pump possesses the advantages of low cost, high efficiency, lightweight, and small volume. Therefore, the performance of a multiphase pump directly determines the extraction efficiency of oil and gas resources. For multiphase pumps, the leakage is one of the key performance indicators, and the magnitude of the leakage directly determines the operational efficiency of the multiphase pump. Mechanical seals are widely used in multiphase pumps due to their excellent sealing performance, long operating cycles, and wear resistance. However, in the context of oil and gas media transportation, it is observed that both the oil and gas phases enter the sealed end face concurrently, resulting in the formation of an oil and gas mixed film. Moreover, methane present in natural gas is susceptible to dissolving or precipitating in the oil phase due to variations in film pressure, thereby influencing the sealing performance. Additionally, fluctuations in gas content can induce shaft vibration and impact operational conditions (such as speed, pressure, and gas content), causing transient alterations in the sealing ring, disturbances in film thickness, increased wear of the seal ring, and heightened leakage. In this paper, a transient dynamic model of the mechanical seal considering methane dissolution was established to investigate the effects of vibration and impact on the transient sealing performance. The evolution characteristics of the liquid film flow field and transient sealing performance under different displacement disturbances, dynamic parameters, and operational fluctuations were presented. The results showed that vibration caused fluctuations in the dynamic pressure of the liquid film and the cavitation zone, with a range between 67.2% and 172%. As the displacement disturbance increased, the amplitude and frequency of the fluctuations in oil film thickness, opening force, and leakage increased, and the phase trajectory became sparser. Compared with Δz=2.0 μm, the time for the sealing system to recover stability at Δz=4.0 μm had increased by 1.5 times. When the O-ring damping was 1.2 kN·s/m and the floating ring mass was 0.5 kg, the fluctuation amplitude and frequency of the sealing performance were minimized. The transient response amplitude and frequency of the sealing performance increased with the increase of rotating speed impact, gas content impact, and pressure impact. The mechanical seal in multiphase pumps should use O-rings with high damping and floating rings with low mass, and the anti-interference capability of the sealing system could be improved by increasing the compensation mechanism. Through the above research, it was possible to understand better the evolution of the liquid film flow field and the sealing performance under different conditions. This was of great significance for improving the design and performance optimization of liquid sealing systems.