Abstract: Microplastic, as an emerging pollutant, has received increasing concerns worldwide. In addition to threatening the health of organisms, microplastics may also affect the nitrogen cycles by altering the colonized biofilms. However, the relevant research is relatively scarce. Based on the microplastic pollution status in different environments, the impact of microplastics on nitrogen transformation in different environmental media (e.g., sludge, water, sediment, and soil) was introduced. Then, the response and transformation mechanism were analyzed in detail. It is found that previous studies mainly focused on the influence impact of microplastics on the nitrogen cycle in sludge and soil, while few of them were conducted in water and sediment. Many influencing factors, including environmental media, polymers, concentration, and size of microplastics, can significantly change the impact of microplastics on the nitrogen cycle. Generally, microplastics can affect the processes of nitrification and denitrification in the environments by affecting the microbial community, enzyme activities, and functional genes related to nitrogen transformation, as well as changing the oxygen flux. The microbial community was mainly affected by the additives released from the microplastics. In addition, microplastics may also promote the growth of the related microbial communities as an organic substrate. Furthermore, microplastics can increase oxygen flux by changing the porosity of the sediment, thereby enhancing nitrification. In the future, more attention should be paid to the impact of microplastics on the nitrogen cycle under different environmental factors or in the critical zones of the earth, such as the hyporheic zone. These results are useful to comprehensively assess the impact of microplastics on ecological and environmental health.