[Objective] Watermelon Fusarium wilt caused by Fusarium oxysporum f. sp. niveum is a common destructive soil-borne disease. Understanding of the competition between non-pathogenic congeneric Fusarium species and pathogenic Fusarium oxysporum strains has contributed to acquisition of novel bio-control agents and broadening the biological control measures against the plant disease. [Methods] Selective media and dilution plating procedure were used to study colonization of bulk soil, rhizosphere soil and plant tissues of watermelon grown in greenhouse pots by non-pathogenic Fusarium verticillioides XA and pathogenic Fusarium oxysporum LD. [Results] Strains XA and LD were isolated from diseased plant tissues in field. When non-diseased soil was solely inoculated with strain XA or strain LD, inoculation with strain XA had no symptom of wilt and no loss of biomass of watermelon whereas inoculation with strain LD led to heavily symptom of wilt. As compared with the treatment of LD, the treatment of dual inoculation with strains XA and LD increased the fresh weight and dry weight of the aerial part of watermelon plants by 151.2% and 110%, respectively. Strain XA successfully colonized roots but was not found in the basal part of stem of watermelon, while strain LD infected plant tissues and soil with (1.58?4.85)×104 CFU/g. As compared with the treatment of LD inoculation, the dual inoculation with strain XA and strain LD decreased the pathogenic F. oxysporum numbers in the basal part of stem, roots, rhizosphere soil and bulk soil of watermelon by 63.3%, 66.1%, 3.3% and 24.4%, respectively and increased the non-pathogenic F. oxysporum numbers in roots, rhizosphere soil and bulk soil of watermelon to (0.35?3.84)×104 CFU/g; this treatment gained 57.8% of control efficiency against watermelon Fusarium wilt. [Conclusion] The non-pathogenic congeneric strain XA effectively reduced the ability of pathogenic F. oxysporum LD to infect watermelon plants and had certain effectiveness as a biocontrol agent against watermelon Fusarium wilt.