Nitrogen (N) is one of the key factors that profoundly impact the global methane (CH₄) metabolism in wetlands. Whether nitrogen input would increase methane emission from wetlands and risk of global climate warming has been paid much attention. However, the magnitude and direction of methane emission rates in response to nitrogen input on a global scale are still unclear. The main study conclusion in the published meta-analysis papers was that nitrogen input increased methane emission from wetlands. But the conclusions also included nitrogen input decrease and did not affect methane emission from wetlands in lots of research papers. Nitrogen input is the driver with the most controversial effects, for which it is difficult to draw univocal patterns. Wetland methane metabolism includes methane production (methanogensis), methane oxidation, methane transport, and their final result of methane emission. This paper reviewed the complexity of effects of nitrogen input on the methane emission from paddy field, inland wetlands, and coastal wetlands. We also analyzed the responses of the rate and pathway of methane production, aerobic methane oxidation, and nitrate/nitrite-dependent anaerobic methane oxidation to nitrogen input, and their mechanism. NO^-₃-N input demonstrated inhibition on methane production, however the effects of input of other forms of nitrogen on methane production showed an uncertainty. The main controlling mechanisms of nitrogen input on methane production included ionic toxicity, promoting plant growth-increasing carbon substrate (such as plant litter, root exudates, sloughed-off cells, and roots etc.) supply, electron acceptors-carbon substrate competition, and pH regulation. The studies of nitrogen input on aerobic methane oxidation mostly focused on the rice fields and peatlands, and the effects included increase, decrease and no-effects on methane oxidations. Nitrogen input increased the nitrate/nitrite-dependent anaerobic methane oxidation. We emphatically analyzed the reasons of different influences of nitrogen input on methane metabolism, and proposed that the response of methane metabolism to nitrogen input was a biogeochemistry process at ecosystem level. We also provided a conceptual model that included microbial processes and ecosystem traits factors to predict effects of nitrogen input on methane emissions from wetlands. Finally, we proposed some basic research domains that needed to strengthen in study of effects of nitrogen input on wetland methane metabolism. Overall, our review shed light on the complexity and mechanism of effects of nitrogen input on methane metabolism in wetlands and will help improve ecosystem models for predicting wetland methane flux caused by nitrogen input.