This paper evaluated the effects of arbuscular mycorrhizal colonization on the growth and drought tolerance of Poncirus trifoliata in a potted culture under natural water stress and resuming water supply. The results showed that the arbuscular mycorrhizal colonization by Glomus mosseae 93 could significantly increase the plant height, stem diameter, and fresh weight of myocrrhizal plant. By the end of the experiment, the survival percentage of transplanted mycorrhizal plant was 7.1% higher than that of nonmyocrrhizal plant. During natural water stress and resuming water supply, the soluble sugar and soluble protein contents and the SOD, POD and CAT activities of root or leaf were higher or significantly higher in mycorrhizal than in nonmycorrhizal plant, suggesting that arbuscular mycorrhizal colonization could improve the osmoregulatory response of myocrrhizal plant, enhance its protective capability, and depress its cell membrane oxidative damage to lipids. Consequently, the drought tolerance of inoculated Poncirus trifoliata was enhanced. It was considered that the functional mechanism of arbuscular mycorrhizal fungus in enhancing the drought tolerance of host plant was closely related to the protective systems such as SOD, POD, CAT, and soluble protein, etc.