Abstract: A greenhouse pot experiment was conducted to study the effects of arbuscular mycorrhizal fungus Glomus versiforme on the seedling growth and root membrane permeability, malondiadehyde (MDA) content, and defensive enzyme activities of non-grafted and grafted watermelon growing on the continuously cropped soil. Inoculation with G. versiforme increased the seedling biomass and root activity significantly, and decreased the root membrane permeability and MDA content. The seedling shoot fresh mass, shoot dry mass, and root activity of non-grafted watermelon increased by 57.6%, 60.0% and 142.1%, and those of grafted watermelon increased by 26.7%, 28.0% and 11.0%, respectively,  compared with no G. versiforme inoculation. The root membrane permeability of non-grafted seedlings (C), grafted seedlings (G), nongrafted seedlings inoculated with G. versiforme (C+M), and grafted seedlings inoculated with G. versiforme (G+M) was in the order of C>G>C+M>G+M, and the root MDA content was in the sequence of C>G>G+M>C+M. G. versiforme inoculation increased the root phenylalanine ammonialyase (PAL), catalase (CAT), peroxidase (POD), β-1,3-glucanase and chitinase activities of grafted and non-grafted seedlings significantly, and the peaks of the POD, PAL and β-1,3-glucanase activities in the mycorrhizal roots appeared about two weeks earlier than those in the non-inoculated roots. These results indicated that inoculating arbuscular mycorrhizal fungus G. versiforme could activate the defensive enzyme activities of nongrafted and grafted watermelon seedlings, enable the seedling roots to produce rapid response to adversity, and thus, improve the capability of watermelon seedling against continuous cropping obstacle.