Abstract: Subsurface flow constructed wetlands have the advantages of simple operation, low cost of construction and operation, and good environmental benefits. However, in the long-term operation, the phenomenon of low denitrification efficiency is also common. The mechanism of denitrification in subsurface flow constructed wetland was stated and the main reasons for the decrease of denitrification efficiency in long-term operation were reoxygenation capacity reduction and substrate clogging. Based on extensive literature review, the methods of improving denitrification efficiency by long-term operation of subsurface flow constructed wetlands were summarized. The measures and effects for improving denitrification efficiency were discussed from the aspects of improving reoxygenation capacity of wetland, selecting suitable filler size and gradation, and optimizing subsurface flow wetland process.