Abstract: To systematically evaluate the feasibility of chitosan oligosaccharides (COS) and chitosan lactate (CHL) in the postharvest disease control of citrus fruit, the direct effects of COS and CHL on the mycelial growth, pathogenicity, spore germination, and spore survival of Penicillium digitatum, Penicillium italicum, Geotrichum citri-aurantii, and Colletotrichum gloeosporioides were investigated. The effects of COS and CHL treatments on the microscopic structure of mycelia and cell membrane permeability of the four pathogenic fungi were also evaluated by scanning electron microscopy and the determination of extracellular relative conductivity, extracellular nucleic acid and protein content. The results showed that the inhibitory effects of COS and CHL on mycelial growth were dose-dependent. The MIC range of COS against the four fungi was all above 20 mg/mL, while the MIC range of CHL against the four types of fungi was between 2 and 20 mg/mL. Following treatment with 20 mg/mL COS and 15 mg/mL CHL, the spore pathogenicity, spore germination rate, and spore survival of the four pathogens were significantly reduced compared to the control group (P<0.05). COS and CHL had a detrimental effect on fungal mycelia, causing wrinkling and deformation, increased the permeability of the cell membrane, and resulting in severe leakage of intracellular components such as nucleic acids and proteins. In summary, COS and CHL have been demonstrated to possess a broad-spectrum direct antifungal ability against four major postharvest pathogenic fungi in citrus. Their utilization, either individually or in conjunction with each other, is anticipated to serve as effective natural antibacterial preservatives. This will provide a feasible alternative method for the routine prevention and control of citrus postharvest diseases.