The biological characteristics of Pythium ultimum causing damping-off disease of Dendrobium officinale and the screening of antifungal plants against P. ultimum were studied to lay a theoretical foundation for the prevention and control of damping-off disease of D. officinale. The inhibitory effect and biological characteristics of aqueous extracts of different Chinese herbal medicines on the  P. ultimum were determined by mycelium growth rate method.The average mycelial growth rate of the  P. ultimum on the PDA medium was 0.22 cm/h.The optimum temperature was 30 °C,the optimum pH was 6-7,the optimum carbon source were glucose and soluble starch,and the optimum nitrogen source was yeast extract. The light has no significant impact on the growth of mycelium. When the concentration of aqueous extract was 40 mg DW/mL,the inhibition rates of aqueous extract of Eugenia caryophyllata Thunb. and Mentha haplocalyx Briq. were 100% and 96.48%,respectively. The water extracts of E. caryophyllata and M. haplocalyx have good application potential in the control of Dendrobium officinale damping-off disease.