In the lyocell process, N-methylmorpholine-N-oxide (NMMO) with a high recovery rate is used as a solvent to directly dissolve cellulose, which is of great significance for the realization of industrialization. In this paper, the impurity removal process and side reactions in the NMMO recovery system are introduced. The NMMO recovery system is generally composed of three major processes: air flotation, ion exchange and evaporation. Air flotation and ion exchange are the processes of removing impurities. And evaporation, where side reactions occur, is the process of increasing the concentration of NMMO. In terms of conductivity and colority, organic flocculants are better than inorganic flocculants in the air flotation process which mainly removes undissolved particulate matter, part of hemicelluloses and some colored substances. The ion exchange process mainly removes ionic substances and propyl gallate (PG). Side reactions occur during evaporation, producing colored substances and black particulate matter. Through analysis, it can be known that the production of colored substances is caused by the condensation reaction of N-methylmorpholine (NMM, 1), morpholine (M, 2), 2-Amino-4-methyl-oxazole (3), furfuryl alcohol (4), Morpholinoacetonitrile (5), xylose and NMMO under alkaline and high-temperature conditions, rather than by PG. The black particles are due to thermal degradation of NMMO or NMM or M in the presence of oxygen and iron at high temperatures.