Roadbed stability, including thermal and strength stability, is the ability of roadbed to adapt to pavement settlement, slurry effusion, shear slide and haunching up of the roadbed resulting from moving vehicles and many natural influences. Thermal stability is the sensitivity of the thermal state of the roadbed to the external environmental condition. It is the key point of the roadbed stability in permafrost. Research concerning roadbed thermal stability in permafrost is analyzed, and the judgment principle of thermal stability is discussed. The regular change of the ground temperature and deformation of particlesize improved roadbeds are analyzed by studying field observation data of the particlesize improved roadbed after two freezing and thawing cycles. We conclude that the particlesize improved roadbed can raise the upper limit of frozen soil. Based on the judgment principle, the particlesize improved roadbed is regarded as a new type of stable structure.