Abstract: In permafrost region, the key about the stability of earth structure is itself thermal stability. If retaining wall would be built in permafrost region, it would change the heat balance of ground layer and the upper limit of frozen soil or depth of seasonal-thawing would change. The stability of engineering structure would be affected by the heat balance change. So it is value to study the development of the upper limit of frozen soil or temperature field distribution according to the seasonal change. In this paper, considering the only retaining constructing - L type retaining wall in section from Golmud to Lhasa, Qinghai-Tibet railway, we test the temperature along the cross-section of retaining wall. We analyze the upper limit of frozen soil or depth of seasonal-thawing of typical backfill. The distribution shape of depth of seasonal-thawing in different cross-sections has been obtained; and the temperature field distribution according to the seasonal change has been regressed also. From testing we know that the ground heat balance has been destroyed by excavation and construction. As an open area engineering structure, the ground heat balance could be recovered gradually if there is no new disturbance. In addition, the effect of freezing force or frozen-heave force shouldn’t be neglected. In a word, the design idea and the engineering measure have been studied in this paper. All these work want to be benefit for future engineering application

Key words: L-type retaining wall, permafrost, thermal stability, depth of seasonal-thawing