In order to explore the gas inerting mechanism and improve the inerting process efficiency, the numerical simulation and optimization of gas inerting process in the horizontal ellipsoid LNG tank is performed. With the gas inlet modes of direct current jet, swirling jet and mixed jet at the end of the ellipsoid tank, the flow field structure and inerting effect of gas jet are analyzed, the influence mechanism of different gas inlet modes on the inerting process is explored, and their inerting optimization schemes are refined. The results show that：the inerting effect of swirling jet is better than that of directcurrent jet and mixed jet when the gas inlet flow is fixed, this is because the swirling jet generates a larger gas inlet divergence angle, which can greatly reduce the presence of inerting dead angle, and is conducive to forming the displacive inerting inside the tank; compared with the directcurrent jet, the swirling jet can save 40.4% nitrogen and inerting time, while compared with the mixed jet, the swirling jet can save 26.2% nitrogen and inerting time. The swirling jet optimization scheme can reduce the nitrogen consumption, save the inerting time, improve the inerting process efficiency, possess the higher economy, and be of important guiding significance for the gas inerting process of actual LNG tank.