The robust warm sea surface temperature (SST) bias in the Southern Ocean (SO) has persisted for decades in several generations of Coupled Model Inter-comparison Projects (CMIP), yet the sources of this bias remain controversial. Here, based on the latest released CMIP6 outputs, we manage to find a common mechanism for this warm SST bias. The warm SST bias consistently features a zonally asymmetrical pattern just along the Antarctic Circumpolar Current (ACC) path. Investigations of its origins show three points: 1) Heat flux bias in CMIP6 acts as a coupled response to dampen warm SST, where shortwave radiation from AMIP6 models appears a negative bias without contribution. 2) AMOC strength and AMOC depth are not common origins. Although there are inter-model correlations existed, they disappear in Ocean Model Inter-comparison Projects. 3) Warm Northern Atlantic Deep Water Mass (NADW) leads to the SO warm SST bias in CMIP6, OMIP1, and OMIP2, through overturning circulation and upwelling to the SO surface.