[Objective] Watermelon is susceptible to infection of Fusarium oxysporum, which causes it to wilt. Colonization of Arbuscular mycorrhizae (AM) on roots of the plants can not only effectively promote plant nutrient uptake, but also help prevent diseases. However, direct inoculation of arbuscular mycorrhizae in the field may in most cases affect its infection efficiency and ecological effects.[Method] In this study, Rhizophagus intraradices (R.i), was inoculated directly to watermelon seedlings in nursery to culture arbuscular mycorrhizae inoculated watermelon seedlings, some of which were then transplanted into pots for observation of how the inoculation helped the seedling to resist wilt disease and for analysis of its mechanism. Meanwhile activities of wilt-resistance related enzymes and their encoded gene expression were determined for analysis of mechanism of AM helping the seedlings resist wilt disease. Besides, some of the inoculated seedlings were transplanted into fields for exploration of effect of the inoculation helping the plants resist wilt disease and improve P nutrition in the field.[Result] Results show that watermelon seedlings inoculated with arbuscular mycorrhizae in nursery maintained their mycorrhizal colonization after transplanting, and even up-regulated the expression of chitinase-encoding genes of ClPR4 and ClPR5 and the expression of β-1,3 glucanase gene ClGlu3 and Phenylanlanine ammonia-lyase encoding gene ClPAL4 and ClPAL11 in their root systems. Chitinase activity in rhizosphere of the watermelons increased significantly, but β-1,3 glucanase and phenylalanine ammonia-lyase activities were unchanged. Meanwhile, the inoculation significantly reduced the number of pathogens in watermelon rhizosphere soil, increased the number of arbuscular mycorrhizal spores, and improved the watermelon root system environment, thus reducing wilt incidence of the plants by 22.22%, and disease index by 23.15%, and consequently increasing the disease controlling effect up to 36.23%. Moreover, the inoculation enhanced the activity of acid phosphatase in the rhizosphere soil, thus increasing the availability of phosphorus in the soils, optimizing phosphorus nutrition of the soil and eventually improving disease resistance of the crop.[Conclusion] When watermelon wilt disease occurs, colonization of AM fungi can increase activities of the disease resistance-related enzymes by increasing the expression of ClaPR4, ClaPR5, ClaGlu3 and ClaPAL4 in the watermelon root system, thereby improving disease resistance of the watermelon; inoculation of seedlings with arbuscular mycorrhizae can improve microbial environment of the plant root, and promote hydrolysis of insoluble phosphorus by stimulating the activity of acid phosphatase in the rhizosphere soil, thus increasing the content of available phosphorus in the soil and promoting absorption of the nutrient by watermelon, and eventually improving disease resistance of the crop, reducing the incidence index of watermelon wilt. It is, therefore, concluded that inoculation of the seedlings of watermelon with arbuscular mycorrhizae can effectively prevent occurrence of Fusarium wilt.