Abstract: In this experiment, one of antagonistic bacteria named a4 was isolated from the soil which collected from the rhizosphere of regular growing medicinal chrysanthemum, its resistance against the pathogen (Fusarium oxysporum f. sp. Chrysanthemi) was screened respectively by the improved confront culture method, and its biocontrol effect on the medicinal chrysanthemum fusarium wilt was examined by the pot experiment and the field plot trials. Results showed: 1)The inhibition rate of a4 against the pathogen was 71%. 2) In the pot experiment, the wilt disease incidence of the treatment adding a4 (CK+BOF1) was 0% compared to 100% in control. The number of the pathogen in rhizosphere soil of control was 6.5 fold higher than that of CK+BOF1. 3) In the field trial, the number of the pathogen was 2103 cfu/g in rhizosphere soil and 2.8 103 cfu/g in the bulk soil in the treatment of (n+F+a4), which were applied BOF in nursery soil and added antagonistic bacterial a4 into transplanted soil, it was significantly lower than control (1.4104 cfu/g in rhizoplane soil and 1.2104 cfu/g in the bulk soil). 4) Biomass of medicinal chrysanthemum was significantly increased in the treatment of n+F+a4 comparing with control, the root biomass increased 92% (fresh weight) and 84% (dry weight); while the fresh weight of shoot increased 77% and dry weight increased 69%. During 120 day experiment period, the control efficiency of n+F+a4 on medicinal chrysanthemum fusarium wilt was 93.2%. and the biomass with it was 24.5 g/plant (DW), was significantly higher than that of control (5.46 g/plant, DW). Results suggested that applying BOF both in nursery and transplanting can effectively prevent the occurrence of medicinal chrysanthemum fusarium wilt disease, promote the growth of medicinal chrysanthemum plant, and increase its yields, thus easing continuous monocropping obstacles.