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Radial oxygen loss (ROL) in plants plays an important role in the oxygen supply of subsurface‐flow constructed wetlands (SSF CWs). In this research, two types of microcosm, Phragmites australis‐planted wetlands (PA) and unplanted wetlands as the control (CT), are set up to investigate seasonal variations in ROL and their effects on pollutant removal. The chemical oxygen demand (COD) and NH4+‐N removal efficiency are detected, and the oxygen contributions produced by ROL are calculated for different seasons. The results show that the removal efficiency differences between PA and CT are significant in autumn and winter. The oxygen obtained from ROL is considerable in spring (120.12 ± 8.85% mg m−3 per day) and summer (1663.3 ± 36.4% mg m−3 per day), followed by autumn (494.32 ± 60.7% mg m−3 per day) and winter (27.95 ± 9.84% mg m−3 per day) and is positively correlated with the removal efficiency of COD (r = 0.786, p < 0.01) and NH4+‐N (r = 0.758, p < 0.01). Investigations of plant growth and physiological indexes showed that ROL is positively correlated with dry weight (DW), net photosynthetic rate (Pn), stomatal conductance (Gs), and especially root porosity (r = 0.731, p < 0.01) and negatively correlated with plant height. These results indicate that the oxygen produced by ROL in CWs is considerable throughout the year, even in winter, and that root porosity has an important impact on seasonal variation in ROL.