Crown rot caused by Colletotrichum siamense is a serious disease of strawberry in the Yangtze River region, China. The metabolites involved in phenylpropanoid biosynthesis increase susceptibility of crown rot in octoploid strawberry. Exogenous coumaric acid, caffeic acid, and ferulic acid involved in phenylpropanoid biosynthesis were used to explore whether the increased susceptibility was associated with reactive oxygen species, antioxidant substance and key genes of phenylpropanoid metabolism. According to the results, H₂O₂, O₂^.−, and MDA contents showed different responses to C. siamense infection in root, petiole and leaf. The H₂O₂, O₂^.− and MDA were increased by C. siamense in petiole. Exogenous coumaric acid, caffeic acid and ferulic acid promoted the H₂O₂, O₂^.− and MDA as positive control. The POD activity was significantly induced in response to C. siamense infection in petiole. The coumaric acid treatment inhibited the POD activity but increased total phenolics. The ferulic acid only promoted POD activity in petiole. In addition, the expressions of transcripts involved in phenylpropanoid biosynthesis were regulated by exogenous coumaric acid, caffeic acid, and ferulic acid. Together, our results demonstrated that exogenous coumaric acid, caffeic acid, and ferulic acid increased susceptibility of octoploid strawberry to crown rot by regulated ROS, antioxidant substance, and transcripts expression of phenylpropanoid biosynthesis.