A prominent feature in the sea ice in the west Cosmonaut Sea is the reoccurrence of a sensible heat polynya—western Cosmonaut Sea Polynya (wCSP), which normally exists in late austral autumn and early winter. In this study, the Southern Ocean Sate Estimate (SOSE) reanalysis product is employed to investigate the thermodynamic and dynamic processes controlling the formation and evolution of a real wCSP event in 2009, when the polynya activities revealed from satellite observations are reasonably simulated by SOSE. Oceanic heat budget analysis was conducted for the surface layer above the thermocline, and the results reveal that heat advection is the major term contributing to the surface heat content variation in this area. The precursor of wCSP—an embayment was formed during the existence of cyclonic atmosphere circulations. The negative wind stress curl from a cyclone induced upwelling which brought the warm circumpolar deep water to the surface. Meanwhile, the location of the cyclone relative to the wCSP created weaker easterlies over the northern boundary of polynya and stronger easterlies over the southern boundary of polynya. This leads to differences in the meridional heat transports across the northern and southern boundaries, resulting in net positive meridional heat transport into the polynya. Both the vertical heat advection and the meridional heat advection contribute significantly to increased heat content in the surface layer of wCSP, causing sea ice melt and formation of the embayment. Meanwhile, the cyclonic wind field promotes convergence of sea ice over the northern area of the embayment and results in a polynya shape. The formation of wCSP is likely related to the strength and position of Southern Annular Mode on the interannual scale, and may potentially affect the biological productivity by controlling the availability of light.