Eutrophication of lakes and aquatic systems deteriorate the ecosystem and economy. The causes and controls of eutrophication have attracted attentions worldwide, including China. Extensive research has been carried out on problems related to eutrophication in China. Nitrogen is one of the major nutriments contributing to the eutrophication in aquatic environments. The external nitrogen load (including point source and nonpoint source) is an important component of the total water-pollutant load. The traditional wastewater treatment technology, which lacks in integrated collection systems, poses excessive investment costs on the control of nonpoint pollution. The constructed wetland is an important technology to reduce the external nitrogen load, especially from the nonpoint source. It is capable of effectively removing nitrogen and handling a high pollutant load; moreover, it is friendly to eco-system and requires low investment costs. Thus the constructed wetland is especially fit for controlling the eutrophication. Illustrating the nitrogen removal mechanisms in constructed wetland has an important meaning on the eutrophication in aquatic systems. The nitrogen removal mechanism in the constructed wetland with a leak-proof layer consists of volatilization, ammonification, nitrification/denitrification, plant uptake, and matrix adsorption, is reviewed in this paper. In a constructed wetland without a leak-proof layer, the nitrogen exchange with the surrounding environments has significant effects on the nitrogen removal rate. Generally, nitrification/denitrification is the major nitrogen removal mechanism in constructed wetlands. Volatilization is insignificant when the pH of the wastewater is less than 7.5. Volatilization is significant when the pH is higher than 9.3. Nitrification/denitrification is the major nitrogen removal mechanism in constructed wetlands treating domestic wastewater. Under low influent loads, suitable climate conditions, and appropriate plant species, the plant-harvest process maybe the major nitrogen removal pathway. Well-managed artificial wetlands usually show a better nitrogen-removing efficiency than do natural wetlands. Sound system designs (e.g., the medium configuration, the arrangement of helophytes, and the opitimization of water distribution and collection) have a direct impact on the nitrogen removal in constructed wetlands. Reasonable operations and maintenances, such as the effective control of the water level, the correct helophyte cultivation, and the sound helophyte harvest can effectively improve the nitrogen removal in a constructed wetland.