In the spring of 2021, the Caulis sinomenii leaves with leaf blight were collected from the Zhengqing Pharmaceutical Base in Zhongfang County, Huaihua City, Hunan Province, and from the experimental farm of Hunan Agricultural University. The pathogen was isolated by tissue separation method and verified by Koch’s rule. The pathogen was identified as Alternaria alternata(A. alternata) by morphological and molecular phylogenetic identification. The mycelium growth rate method was used to determine the inhibition effect of 5 agents(95% Difenoconazole, 98% Prochloraz, 96% Hexaconazole, 96.5% Prothioconazole and 98% Azoxystrobin) to the pathogen, and the result showed that 95% Difenoconazole exhibited the best inhibitory effect with the EC50 of 0.043 6 mg/L, while 98% Azoxystrobin has the worst control effect with the EC50 of 8.447 3 mg/L.