To develop endophytic fungi resources from medicinal plants, this paper studied the correlation between endophytic fungus Fusariium sp. ES from Euphorbia pekinensis and its host through analyzing the inhibition activity of this pathogen. An endophytic strain S12 (Alternaria sp.) from Sapium sebiferum was used as contrast. The results showed that E5 had an inhibitory effect on the host, while S12 and E. pekinensis extract didn't have it. The inhibition activity of E5 could be stimulated by the extract of host leaf, and its inhibitory effect could only be found when it was cultured with enough oxygen. The inhibition activity was affected by pH, temperature, and ultra-violet. These characters of endophytic fungus E5 showed a good correlation with its host, and this correlation was mutualism.