Due to its low cost and energy consumption and more significant functions, constructed wetlands (CWs) have always been regarded as sustainable wastewater treatment technologies. However, CWs may also cause a secondary pollution to the environment because of its considerable emission of greenhouse gases (GHGs),including CO_2,CH_4 and N_2O, and are under suspicion of transferring water pollution to air pollution. Under the circumstance of signing the Paris Agreement whose core is controlling climate change, more attention should be paid to CWs' GHGs emission. The mechanisms and characteristics of GHGs emission in different types of CWs were introduced and analysed,followed by the comparisons between CWs and conventional wastewater treatment processes. The influences of wetland plants, substrates, season, influent loading rate and hydrological conditions on GHGs emission were discussed. Finally, several technical approaches to control GHGs emission from CWs were recommended.