Abstract: 【Objective】The study was to investigate the effect of polysiloxane-polyacrylate quaternary ammonium salt block copolymer (Six-Q5) on cell membrane of Rhizoctonia solani, and to provide a new idea for the control of rice sheath blight.【Method】The effects of Six-Q5 on cell membrane integrity of R. solani were observed under differential interference microscopy after PI staining. The effects of Six-Q5 on cell membrane permeability of R. solani were characterized by Coomassie brilliant blue method, anthrone-sulfuric acid method and conductivity method, respectively. Malondialdehyde (MDA) kit was used to determine the content of MDA to characterize the effect of Six-Q5 on lipid peroxidation of cell membrane of R. solani.【Result】Under the fluorescence microscope, the mycelia treated with Six-Q5 are observed with fluorescent bright spots. It is found that the protein, carbohydrate and electrolyte contents in the mycelium culture medium of the treatment are significantly different from those of the control. The MDA content of mycelia and culture medium in the treatment increased with the increas of drug concentration. With the difference of PDMS block length, its influence on the R. solani mycelia membrane is also different.【Conclusion】The results show that Six-Q5 destroy the integrity of cell membrane, chang the permeability of cell membrane and the structural and functional integrity of cell membrane. Moreover, with the increase of the block length of Six-Q5, the effect is more pronounced. Cell membrane is a target of Six-Q5 for inhibiting rice sheath blight.