Abstract: 【Objective】This study provided a reference for the selection of characterization methods of amphiphilic macromolecule quaternary ammonium salts (QAS) on plant pathogenic fungi.【Method】The antifungal activity of new amphipathic macromolecular QAS copolymers 〔PDMS-b-(PDMAEMA-BC), labeled as Six-Q5〕, water soluble macromolecule QAS (PQD-BC) and small molecular QAS (benzalkonium chloride, BC) against Rhizoctonia solani were characterized by growth rate method and mycelium weighing method respectively.【Result】Growth rate method can be used to characterize the antimicrobial activity of hydrophilic macromolecular QAS (PQD-BC) and small molecular QAS (BC), but not used to characterize amphiphilic macromolecular QAS. Mycelial weighing method can be used to characterize the antimicrobial activity of three kinds of QAS. The IC50 and IC90 of Si5-Q5 with moderate length of PDMS block for R. solani were 12 μg/mL and 59 μg/mL respectively, and its antifungal activity was similar to that of small molecular QAS. 【Conclusion】 For amphipathic macromolecule QAS, mycelial weighing method was more suitable than growth rate method for the characterization of antifungal activity.