Ammopiptanthus mongolicus, a xerophyte plant that belongs to the family Leguminosae, adapts to extremely arid, hot, and cold environments, making it an excellent woody plant to study the molecular mechanisms underlying abiotic stress tolerance. Three dehydrin genes, AmDHN132, AmDHN154, and AmDHN200 were cloned from abiotic stress treated A. mongolicus seedlings. Cytomembrane-located AmDHN200, nucleus-located AmDHN154, and cytoplasm and nucleus-located AmDHN132 were characterized by constitutive overexpression of their genes in Arabidopsis thaliana. Overexpression of AmDHN132, AmDHN154, and AmDHN200 in transgenic Arabidopsis improved salt, osmotic, and cold tolerances, with AmDHN132 having the largest effect, whereas the growth of transformed plants is not negatively affected. These results indicate that AmDHNs contribute to the abiotic stress tolerance of A. mongolicus and that AmDHN genes function differently in response to abiotic stresses. Furthermore, they have the potential to be used in the genetic engineering of stress tolerance in higher plants.