A ground temperature section with thawed interlayer within an embankment along the Qinghai-Tibet Railway had been monitored from 2005 to 2015. Thus the annual variation of thermal state under original state and within embankment,variation of thawed interlayer and its effect on thermal state of permafrost are analyzed in the background of climate warming and engineering activity. The results show that: 1) permafrost table under original state and within embankment and its thermal stability had descended annually; 2) thawed interlayer had existed under left shoulder and its thickness had increased undulately,due to the artificial permafrost table going down; meantime,thawed interlayer had not found under original state and right shoulder; 3) thermal accumulation nearby permafrost table had been remarkable,resulting in decline of permafrost table and increase of temperature nearby permafrost table; moreover,thermal stability of permafrost had been weakened; 4) reinforcement of embankment with active-cooling had successfully improved thermal stability of the embankment; artificial permafrost table had lifted upwards within right shoulder after reinforcement of embankment and its thermal stability had enhanced; whereas,left shoulder had only maintained original thermal stability by the same engineering operation due to the existence of thawed interlayer,then the efficiency of active-cooling had performed poorly under left shoulder to a certain extent.