The eastward moving 4-day wave in the Antarctic winter mesosphere is investigated using a high-resolution middle atmosphere general circulation model that directly simulates the spontaneous generation, propagation, and dissipation of gravity waves. The results are also compared with the simulated seasonal march of the meridional structures of the westerly jet streams in the Southern Hemisphere upper stratosphere and mesosphere in order to investigate baroclinic/barotropic instability as a possible excitation mechanism for the 4-day wave. The model successfully reproduces the dynamically unstable double-jet structure of the mesospheric westerly winds. The simulated 4-day wave develops in association with the baroclinic and barotropic instability of the mesospheric mean flows and has similar characteristics to those observed at 50–60°S near the stratopause. The 4-day wave has strong equatorward heat flux associated with strong baroclinicity in the Antarctic winter mesosphere, mainly attributable to poleward overturning circulation driven by gravity wave forcing. Eastward forcing due to the 4-day wave occurs within the double-jet structure and offsets part of the westward forcing due to gravity waves. Such an effect partially acts to stabilize the unstable mean flow structure in the Antarctic winter mesosphere.