This study aimed to isolate, purify and identify phenols from the celery leaves and further evaluate the antioxidant activities of the monomers in chemical system. Seven polyphenol monomers (a-g) were identified by HPLC-ESI-MS/MS after being separated by D101 macroporous resin, semi-preparative reversed-phase high-performance liquid chromatography (RP-HPLC) and sephadex LH-20. Interestingly, chrysoeriol 7-O-glucoside was identified from celery leaves by UV, IR, one dimensional nuclear magnetic resonance (NMR) and two-dimensional NMR for the first time. In addition, these polyphenol monomers manifested desirable DPPH• scavenging capacity (EC50: 39.55 ± 1.43–612 ± 8.45 μM) and ABTS+• scavenging capacity (EC50: 32.12 ± 0.58–512 ± 25.12 μM), respectively. Among them, the polyphenol monomers (a and b) with two hydroxyl groups (-OH) on the B ring in flavone skeleton had the strongest antioxidant activities. Moreover, the analysis of the structure-activity relationship showed that the position and quantity of -OH on the B-ring and the methylation degree of 3-OH played an important role in regulating antioxidant activities.