The Mahan Mountain is the only region in the loess plateau where permafrost still exists, and the residual permafrost has been called "live fossil" of permafrost. In the past 20 years, permafrost in the Mahan Mountain has accordingly degenerated badly. At present, island permafrost still remains only in Xiaohutan, which is characterized by typical high\|temperature permafrost. Scattered permafrost, found in other places, has generally disappeared since the beginning of the 1990s. The lowest ground temperature of island permafrost in the Mahan Mountain is -0.2 ℃ at the depths between 10 m and 16 m, increasing upwards and downwards with a gradient about ±0.01 ℃·m-1. The annual temperature increasing rate is 0.006~0.012 ℃·a-1, which is less than that in the Tibetan Plateau. The maximum depth of permafrost in the Mahan Mountain is about 40 m, which is undergoing upward and downward degradation. The lateral degradation in the edge regions of the island permafrost is dominant. Abundant segregated ice exists in the permafrost region. From the analysis on development characteristics of ground ice and the determining of 14C from the buried organic matter, it is believed that after the Neoglaciation many permafrost\|accumulations took place in the Mahan Mountain. Symbiosis is the principal reason for the formation of underground ice. In this paper, it is suggested that the main causes for permafrost remnant in the Mahan Mountain are the protective effects from the abundant ground ice and organic matter, as well as the local colder micro\|climate environment.