In order to know the inhibition mechanism of ethyl acetate extract of Trichoderma harzianum fermentation liquid on the growth of Phytophthora infestans, the effects of the extract on pathogen physiological indexes, such as protective enzyme activity, cell content of soluble protein, malondialdehyde (MDA) content, electrical conductivity and respiration rate, were studied by biochemical method. The results indicated that the pathogen’s protective enzyme activities decreased significantly after treated by extract. Forty-eight hours after treatment, the enzyme activity of SOD, CAT, POD and GSH-Px decreased by 89.06%, 86.96%, 97.47% and 97.63% respectively, content of soluble protein reduced by 62.5%, content of MDA increased by 60.98%, electrical conductivity increased by 40.89%, and respiration rate decreased to 0. In conclusion, ethyl acetate extract which was extracted from the fermented liquid of T. harzianum reduced the pathogen’s soluble protein content. Protective enzyme (SOD, CAT, POD and GSHPx) activity and respiration rate were also reduced. Ethyl acetate extract was also found to increase MDA content and electrical conductivity. This means the extract can cause the systems of P. infestans to be changed through, for example, scavenging free radicals, cell membrane damage and electrolyte exposure, resulting in the P. infestans body being damaged. This may be one of the inhibiting mechanisms of ethyl acetate extract of T. harzianum to the growth of P. infestans.