Abstract: The paper aims to clarify the joint effect and optimum ratio of mixed Polyhexamethylene biguanidine hydrochloride (PHMB) and epoxiconazole on Alternaria alternata f. sp. mali. The authors tested the toxicity of mixed PHMB and epoxiconazole on A. alternata by mycelium growth rate method. The results showed that: EC 50 value of PHMB and epoxiconazole on A. alternata was 30.604 and 0.621 mg/L, respectively, when 7 different mixture ratios of PHMB and epoxiconazole were 10:1, 7:1, 4:1, 1:1, 1:4, 1:7 and 1:10, the synergistic coefficients were 1.41, 1.55, 1.59, 1.57, 1.09, 1.12 and 1.14, respectively; the indoor toxicity of epoxiconazole on A. alternata was more active than that of PHMB. When the mixture ratios of PHMB and epoxiconazole were 7:1, 4:1 and 1:1, the synergistic coefficients were all greater than 1.5, so the 3 ratios of ombination all had synergistic effects on A. alternata.