Abstract: 【Objective】To explore the microecological mechanism of biochar on soil improvement by analyzing the growth of ratoon sugarcane soil nutrition and microbial diversity,so as to lay a theoretical foundation for application of biochar in sugarcane cultivation,the alleviation of ratoon sugarcane diseases and the reduction of obstacles to continuous cropping of sugarcane.【Method】Sugarcane in the fourth year of planting was used as the experimental material and two treatments were set up. They were CK(without biochar)and biochar(3 t/ha). The effect of the treatments on soil nutrients,enzyme activities,microbial diversity and sugarcane growth were measured.【Result】The application of biochar significantly increased the contents of alkali hydrolyzable nitrogen,available potassium and organic matter(P<0.05,the same below)compared with CK. Moreover,biochar also significantly increased the activities of sucrase and acid phosphatase. The application of biochar can also change the total number of bacterial and fungal OTUs in soil. The relative abundance of dominant bacterial phylum and genus in soil were equivalent between CK and biochar treatment. Biochar treatment decreased the relative abundance of Ascomycota,Basidiomycota,Amyloflagellula and Fusarium,while increasing the relative abundance of Zygomycota,Chaetomium and Mortierella. In terms of sugarcane growth,the application of biochar significantly reduced the incidence rate of pokkah boeng disease,and the sugarcane yield was significantly increased by 5.2%.【Conclusion】The application of biochar can improve sugarcane soil,change the structure of the soil microbial community,increased the sugarcane resistance to Pokkah boeng,thus increased the sugarcane yield of ratoon sugarcane in the fourth year on dry slope land.