Abstract: Microbial preparation refers to the living microorganism as the main component, including inert carrier materials, nutrients, and other accessory ingredients. It is necessary to maintain the vitality and effectiveness of microorganisms in the biocontrol preparations for a long time, particularly in the development of biocontrol products. The storage stability of biocontrol agents can be a key factor to ensure the number of microorganisms and the biological control. Metschnikowia citriensis can be expected to efficiently control the postharvest green mold and sour rot caused by Penicillium digitatum and Geotrichum citri-aurantii on the citrus fruit, indicating its great application and development value. This study aims to develop antagonistic yeast biocontrol agents with a long shelf life and stable biocontrol effect. The wettable powder was also prepared with the M. citriensis as the main active ingredient. The single factor and mixing tests were carried out to optimize the accessory ingredient for the wettable powder. Then, the wettable powder was applied to the citrus fruits. An accelerated storage test was conducted to predict the storage stability of the preparation, and evaluate the control effect of the preparation on the main postharvest diseases of citrus under in vitro and in vivo conditions. The results showed that the carriers, wetting powder, and suspending agent were screened by the single factor test with the mixing test. An optimal combination of the preparation was achieved with the honeylocust powder of 5.69%, dispersing agent NNO of 8.74%, and freeze-dried powder of 85.57%. The number of viable yeast in the freeze-dried preparation reached 4.72×108 CFU/g, and the wetting time was (7.82±1.02) s, while the suspensibility was up to 71.56%±0.06%. The accelerated storage test demonstrated that better storage performance was achieved in the freeze-dried bioformulation. Among them, the wettable powder was stored at four temperatures. The deactivation rate constant of yeast gradually increased in the wettable powder, whereas, the death rate of yeast accelerated with the increase in storage temperature. The viable yeast population in the preparation was estimated to be 4.6×108 CFU/g, when the storage at 4℃ for one year. Therefore, there was an appropriate reduction of storage temperature for better storage ability. In vitro tests and fruit tests were carried out to learn the application effect of the preparation. The experiments showed that the wettable powder effectively controlled the occurrence of postharvest citrus fruit diseases. It was found that there was no significant change in the inhibitory effect of wettable powder on the postharvest pathogen and pigment production capacity of M. citriensis, compared with fresh yeast. Specifically, the inhibition zone reached more than 9 mm. Fresh yeast presented a remarkable control effect on the blue and green mold, and sour rot on the citrus fruits. The formulation reduced the incidence of three citrus postharvest diseases by 40%-70%. To sum up, the preparation treatment of M. citriensis can be expected to effectively retain cell viability and biocontrol efficacy. The wettable powder with the M. citriensis as the main active ingredient indicated a significant decrease in the incidence of postharvest disease on the citrus fruits. The finding can provide the theoretical and practical basis for the application of M. citriensis in the biological control of postharvest citrus diseases.