This paper used watermelon variety Qianli No. 1 as experimental material,set four biomaterial carbon application levels of 0%(CK),0. 5%(T1),1%(T2) and 2%(T3),and studies the effects of biomaterial carbon application rates on Fusarium wilt of watermelon seedlings and its mechanism.The results showed that biomaterial carbon could significantly reduce the disease index of watermelon Fusarium wilt and improve the control effect. Compared with CK,on the 20th day after inoculation,the disease index of watermelon seedlings treated with T1, T2 and T3 significantly decreased by 10.02%, 19.61% and 29.62% respectively,and the control effects increased by 48.83%,62.66% and 77.25% respectively(P<0.05). At the same time,with the increase of biomaterial carbon application rate,the Fusarium wilt disease index,the number of fungi,the number of Fusarium oxysporum and the content of MDA in watermelon seedlings decreased gradually,while the control effect of Fusarium wilt,biomass of watermelon seedlings,total soil microorganisms number,bacteria number,actinomycetes number,bacteria/fungi,actinomycetes/fungi,protective enzymes(superoxide dismutase, peroxidase, catalase and phenylalnine ammonialyase)and the activity of disease resistance related enzymes(chitinase and β-1,3-glucanase) increased gradually.In conclusion,the external application of biomaterial carbon could effectively improve soil microbial community structure,reduce the number of Fusarium oxysporum,promote the growth of watermelon seedlings,increase the activities of protective enzymes,and disease resistance related enzymes,so as to enhance the resistance of watermelon to Fusarium wilt.