With the increase of watermelon cultivation area and continuous cropping, the harm of watermelon sclerotiniosis is becoming more and more serious. It has now risen to an important disease in watermelon production, which seriously affects the quality and yield of watermelon. The pathogen of watermelon Sclerotinia sclerotiorum is [Sclerotinia sclerotiorum(Lib.) De Bary], which is widely distributed in the world, causes plant sclerotia worldwide. The host range of Sclerotinia sclerotiorum is very wide, which not only harms watermelon, but also infects 75 families, 278 genera and 450 species of Cucurbitaceae, Leguminosae, Solanaceae and Cruciferae. To provide a new control method for the biological control of sclerotinia sclerotiorum in watermelon, the biocontrol bacteria in soil were isolated, screened and identified, and the bacteriostasis was studied. (1) The isolation, identification and biological characteristics of the pathogenic bacteria were determined. The fungus strain LY 24 was obtained from watermelon stem and vine infected with Sclerotinia sclerotiorum. The fungus strain LY 24 had good pathogenicity to watermelon plants by pathogenicity. The strain LY 24 was identified by morphology and molecular biology. The results showed that the best carbon source of strain LY 24 was mannitol, the best nitrogen source was tryptone, the best inorganic salt was NaCl, the best pH value was 9.The best growth temperature was 25℃, and the best light condition was whole darkness. (2) The isolation and identification of biocontrol bacteria in soil were carried out in the watermelon planting base around Changchun, such as the watermelon planting base in Jiutai District. Forty soil samples around the rhizosphere of watermelon plants were collected, and 300 strains of bacteria were isolated from the samples. The bacterial strains with significant antagonistic effect were obtained by plate confrontation method, and the strain XJ-04 with good antibacterial effect was selected as the research object. The inhibitory band width of strain XJ-04 was about 5.21 mm. The inhibitory rate of strain XJ-04 against Sclerotinia sclerotiorum was 70.12%. After morphological and molecular biological identification, strain XJ-04 was identified as Bacillus marisflavi. The physiological and biochemical characteristics of biocontrol strain XJ-04 were analyzed by detecting Gram reaction, contact enzyme reaction, methyl red reaction, V-P reaction and hydrogen sulfide reaction. (3) Studies on fermentation optimization of strain and fermentation broth stability. The optimum composition of culture medium was determined by orthogonal test, which was No.8 culture medium (sucrose, fine bran, K2HPO4·3H2O, pH 9). The inhibition rate was about 71.75%. The optimum addition amount of the above three components was determined by response surface methodology. When the addition amounts of sucrose, fine bran and K2HPO4·3H2O were about 21.08 g/L, 9.17 g/L and 9.77 g/L respectively, the highest inhibition rate was about 77. 78% by Design Expert software, which was about 7.66% higher than that before optimization, and 4% higher than that before optimization. The optimal fermentation conditions were 100 ml of liquid, 30℃ of temperature and 3d of fermentation time. The stability study showed that the bacteriostasis rate of fermentation broth treated by different temperature, pH, storage time and light had little change.