Apple replant disease (ARD) is a biological syndrome that occurs in sites where the same or closely related tree species have been planted previously. It is common in all major apple-growing regions of the world. The problems caused by ARD are typically expressed as reductions in plant growth and development, inhibition of root system development, with a subsequent shortened productive life and reduced yields. Research suggests that ARD is complex and can be induced by many factors. Excessive accumulation of phenolic substances is one of the most important factors. Therefore, studying phenolic acid inhibition of apple trees would provide a scientific and theoretic basis for understanding ARD and enrich research on the continuous cultivation of apple trees. To understand the phytotoxic mechanisms induced by phenolic acids involved in this phenomenon, Malus hupehensis Rehd. seedlings were planted in sand and treated with five phenolic acids (phloridzin, phloretin, cinnamic acid, p-hydroxybenzoic acid, and phloroglucinol) at the same concentrations as found in orchard soils. The effects of these phenolic acids on the function of mitochondria and antioxidant systems of Malus hupehensis Rehd. seedlings were analyzed by measuring plant growth, activity of mitochondrial permeability transition pores (MPTPs), membrane electric potential and cytochrome c/a superoxidase (SOD), catalase (CAT) and peroxidase (POD) activity, along with malondialdehyde (MDA), hydrogen peroxide (H₂O₂), and superoxide radical (O₂^{· -}) content. All five kinds of phenolic acids inhibited the growth of replanted seedlings, and reduced total root length and average root diameter. Phloridzin had more significant inhibitory effects than other phenolic acids, reducing under-ground dry weight, aboveground dry weight, total root length, and average root diameter by 56.5%, 32.9%, 31%, and 27.9%, respectively. Root/shoot ratios were significantly decreased, which indicates that the impact on roots was more serious than on shoots. The phenolic acids increased the opening of MPTPs, decreased membrane electric potential and cytochrome c/a. Furthermore, POD, SOD, and CAT activity declined, which could be responsible for H₂O₂, O₂^- and MDA accumulation under the phenolic acid stress. Phloridzin was more toxic to seedlings than the other four phenolic acids;it reduced SOD, POD, and CAT activity by 29.6%, 16.4%, and 27.5%, respectively, and increased MDA, H₂O₂, and O₂^·- content by approximately 6.3, 6.0, and 1.9-fold compared to the control. Based on the above results, it could be concluded that phenolic acids induce ROS generation, and reduce antioxidant enzyme activity, thereby inducing mitochondrial permeability transition (MPT), and releasing cytochrome c to the cytosol. Therefore, phloridzin is the main phenolic acid occurring in apple continuous cropping orchard soils, and the degradation of phloridzin is the key to alleviating ARD.