Abstract：The excavation of tunnel will affect the thermal stability of surrounding rock in permafrost region. Accordingly，an active layer close to the supporting system of tunnel is formed in the surrounding rock under seasonal thawing and freezing. Based on the excavation of Fenghuoshan Tunnel of Qinghai—Tibet Railway，a detailed experimental investigation is conducted. Some useful conclusions of the active layer¢s thermal regularity are achieved. First，an important fact of in-situ experiment is that temperature of frozen rock behind the liner varies with time and space linearly. Second，another interesting phenomenon is that thawing depth of surrounding rock surpasses the natural thawing depth of this region during construction period. For instance，some areas¢ thawing depths are found below 5 m，while the natural value of this region is only 1.36–2.11 m. Analytical result indicates that the temporary heat produced during excavation causes the additional thawing of frozen rock/soil behind liner，such as chamber blasting，machine operating，construction illuminating，and so on. According to the in-situ data，regression analysis shows that the ratio of thawing depth value during construction and its natural thawing value of this region vary with the ratio of atmospheric temperature inside to outside of the tunnel linearly. Finally，the achieved data illustrate that thawing depth is less than that of the region¢s natural value after tunnel excavation，which is deemed as a vital fact：the special insulating layer which is set between the primary support and the liner structure. Similarly，regression analysis shows that the ratio of thawing depth after excavation to its natural value changes with the ratio of atmospheric temperature inside to outside of the tunnel linearly，i.e. the atmospheric temperature in the tunnel refers to surface temperature of surrounding rock，where temperature monitoring units are installed on the surface of the primary support behind insulating layer.
