Abstract: The leaf spot caused byPhoma medicaginisis a common disease of alfalfa, which usually cooccurs with pea aphid (Acyrthosiphon pisum). To clarify the regulation role of AM fungi, we investigated the defense mechanism of AM fungiinoculated plants after infected byP. medicaginis, and the subsequent damage of pea aphids to plants. The results showed that AM fungi significantly reduced plant disease index (P<0.05). AM fungi regulated the response of plants to leaf spot and aphids by promoting plant growth (P<0.05), changing activities of plant defensive enzyme superoxide dismutase (SOD) and catalase (CAT) and the concentrations of salicylic acid (SA) concentration (P<0.05). Infection of pathogen enhanced the activities of plant antioxidant defense system, including peroxidase (POD), SOD, CAT and PPO (P<0.05), thereby increasing the resistance of plants to subsequent aphids feeding. AM fungi played an active role in defence against pathogen, significantly increased the SOD and CAT activities (P<0.05), and inhibited the damage of leaf spot by P. medicaginis. The aphids further aggravated plant damage and reduced the regulative role of AM fungi and the resistance of alfalfa to pathogen infection.