The antifungal activity of silicon (Si) on Penicillium digitatum, and the possible action mode involved were investigated. Spore germination, germ tube elongation, and mycelial growth of P. digitatum were strongly inhibited by Si in the form of sodium silicate. Using propidium iodide (PI) stain combined with fluorescent microscopy, it was found that the plasma membrane of Si-treated P. digitatum spores was obviously damaged, and the leakage of protein and sugar was significantly higher in Si-treated mycelia than that of control. These findings suggest that the damage on plasma membrane of P. digitatum played a crucial role in the antifungal effect of Si. Moreover, Si was effective in controlling green mold caused by P. digitatum in citrus fruit. These results have a beneficial impact on the application of Si in the control of postharvest diseases.