Abstract: To explore the antifungal function and application potential of Cymbopogon citratus essential oil in the biological control of plant diseases, the disk-diffusion and fumigation methods were used to measure the inhibitory effect of C. citratus essential oil against Alternaria solani and its effect on cell membrane permeability, soluble protein and malondialdehyde (MDA) content, nutrient absorption, and superoxide dismutase (SOD) activity of mycelium. The results showed that C. citratus essential oil had a strong time and dose-dependent inhibitory effect, however showed no sterilization effect on A. solani. Overall, fumigation was a more effective method than disk-diffusion. After 48 hours of C. citratus essential oil treatment, the half maximal inhibitory concentration (IC₅₀) values were 13.40 μL·L^-1 and 103.23 mg·L^-1 for fumigation and disk-diffusion, respectively. After 144 hours, they were 33.81 μL·L^-1 and 145.16 mg·L^-1, respectively. The conductivity and MDA content of mycelium were 2.7 and 2.2 folds as that of the control 12 hours after C. citratus essential oil treatment of 125 mg·L^-1, the SOD activity and soluble protein content were increased by 88.5% and 21.9%, and the absorption of reducing sugar was decreased by 11.3%. These results indicated that C. citratus essential oil could inhibit the growth of A. solani mycelium by destroying cell membrane integrity and reducing nutrient absorption, which should be taken as a potential biological agent for controlling plant diseases.