Fusarium oxysporum is the pathogen of tomato fusarium wilt. In this research, a series of experiments were carried out by Trichoderma koningiopsis T51 strain against F. oxysporum to investigate the biological control mechanism of T51 strain, including confrontation culture test, the screening test of optimum temperature and pH for the prevention, and biological control effect test, where we tested the physiological and biochemical indexes of tomato seedling leaves, and tested the expression of key genes of synthesis and signaling pathways of salicylic acids and jasmonic acids. The results showed that: (1) T51 strain could inhibit the growth of F. oxysporum in confrontation culture significantly, and the inhibition rate of T51 strain against F. oxysporum increased first and decreased then, with increasing ambient temperature and pH, but the best inhibition was achieved at 20 ℃ and pH=7. (2) Tomato seedlings treated with T51 strain and F. oxysporum conidia fluid showed no conspicuous symptom, and there was no significant difference in plant growth status with the control group. Compared with F. oxysporum conidia fluid treatment group, the incidence and disease index were decreased significantly, and the relative control effect was 87.5%. (3) Compared with F. oxysporum conidia fluid treatment, chlorophyll fluorescence parameters, including maximum photochemical efficiency, photochemical quenching coefficient and apparent photosynthetic electron transport rate, and antioxidant enzymes SOD, POD, CAT activity of tomato seedlings inoculated mixed conidia fluid were increased significantly, but non photochemical quenching and the content of hydrogen peroxide were reduced significantly. (4) Compared with F. oxysporum conidia fluid treatment, the content of endogenous salicylic acid (SA) in leaves of tomato seedlings treated with mixed spore solution was decreased significantly, while jasmonic acid (JA) increased significantly. While the expression of PR1 and TGA2 genes in SA signaling pathway were downregulated significantly, and the expression of JA synthesis gene LoxD was upregulated significantly. In conclusion, T. koningiopsis T51 had a significant control effect on F. oxysporum, and the optimal control effect was obtained at 20 ℃ and pH=7. T51 strain may enhance tomato wilt resistance by inhibiting F. oxysporum infecting, enhancing antioxidant enzyme activity, improving photosynthetic efficiency of tomato leaves, regulating the expression of key genes in salicylic acid and jasmonic acid synthesis and signaling pathway, and regulating the contents of endogenous SA and JA. These results provided theoretical and research basis for the biocontrol of tomato fusarium wilt.