Abstract: Thermal behavior of vacuum concentric tube annulus is a process involving combined heat transfer mode,in which the thermal conduction of low pressure gas plays an important role.Accurate description is proposed for thermal radiation and gas conduction in the annulus under various heat insulation conditions and theoretic model is derived.With vacuum outgassing taken into acount,the equivalent conductivity of annulus is analyzed in detail.The method and technique employed is applicable to similar circumstances in the fields including space thermal physics.