Abstract: 【OBJECT】To study the distribution of Fusarium oxysporum inside watermelon plant and its development rule.【METHOD】The experiment was conducted in the watermelon field at Yongtai, Fujian, China. The growth states of watermelon plants with different disease degrees of wilt disease were investigated, as well as the distribution of F. oxysporum at the rhizospheric soils and inside the plants.【RESULTS】As disease developing, it influenced significantly the growth of the watermelon plants. F. oxysporum existed both at the rhizospheric soils of ill and healthy plants, while no significant difference was found among the fungi’s numbers. However, F. oxysporum was detected only at the root of the healthy plants, whereas they were found at both the root and the bottom of stem of the plants at the early stage of the disease, as well as at the root, bottom of stem and the middle part of stem of the plants at the middle stage of the disease. The numbers of F. oxysporum were 3.97、2.07 and 0.27×103cfu/g inside the root of plants at the middle stage of disease, the early stage of disease and the healthy plants, respectively.【CONCLUSION】The number of F. oxysporum inside the watermelon plant related positively to the disease degree, and the fungus had a developing continuity from the bottom to the top.