OBJECTIVE To characterize the chemical constituents of the cladodes and fruits of Opuntia Milpa Alta by using UPLC-QE-Orbitrap MS, and investigate the antioxidant activity, and explore the relationship between the constituents and biological activity. METHODS The UPLC HSS T3 C18 column(2.1 mm×10 mm, 1.8 μm) was used. Acetonitrile and water with 0.1% formic acid was used for gradient elution at a 0.2 mL·min–1 flow rateas. Heated electrospray ion source was used for primary and secondary mass spectrometry data acquisition in positive and negative ion modes, respectively. Database and literature reports were used to characterize the chemical constituents of the cladodes and fruits of Opuntia Mipa Alta. DPPH radical, hydroxyl radical(·OH) and superoxide anion radical(· ) scavenging ability were used to investigate the antioxidant activities of the cladodes and fruits of Opuntia Milpa Alta. RESULTS A total of 39 compounds were characterized from the cladodes and fruits of Opuntia Milpa Alta, including 22 phenolic compounds, 13 flavonoids and 4 other components. Both cladodes and fruits of Opuntia Milpa Alta had certain antioxidant capacity. The fruits of Opuntia Milpa Alta had better scavenging ability of DPPH than the cladodes, while the cladodes had a slightly stronger scavenging ability of ·OH than the fruits. The scavenging ability of · between cladode peels and fruit peels was not significantly different, and the juice had the strongest scavenging ability of · . The antioxidant activity of Opuntia Milpa Alta was closely related to its rich phenolic acids. CONCLUSION In this study, a rapid, accurate and reliable method is established to identify the chemical constituents of the cladodes and fruits of Opuntia Milpa Alta, which provides scientific basis for the comprehensive development and utilization of Opuntia Milpa Alta.