Abstract: The aim of this study was conducted to explore the evaluation method of synegism of fungicide interaction with Trichoderma to plant disease. The inhibitory effect of T. harzianum interaction with five different mechanism fungicides to Botrytis cinerea was respectively determined with confrontation, spore suspension and the detached leaf method. Verification the synegrism through interaction coefficient beyond one in plate, simultaneously, the EC50 differences multiples beyond two of protective effect of interaction and the EC50 differences multiples beyond ten of therapeutic effect of interaction in detached leaf. The control efficiency of fungicide interaction with Trichoderma to gray mold higher than fungicide and Trichoderma used alone by three methods, using interaction coefficient valuation of the synergistic effect was boscalid, azoxystrobin, fludioxonil, SYP-048 interaction with Trichoderma, except fluazinam, and consistent of validation by detached leaf. Interaction coefficient as a evaluation standard of synergistic effect of T. harzianum interaction with chemical pesticides were scientific and reasonable, it could be used as the base of application for interaction control of B. cinerea in production.