Abstract: To study the effects of nitrogen preserving agent (NPA) on composting process and nitrogen loss of Eichhornia crassipes, an aerobic composting was conducted for 35 days using four treatments. The NPA was prepared by mixing ferrous sulfate, humic acid sodium, and superphosphate (M:M:M=75:20:5). Four treatments were included with different mass ratios of NPA, including 0% (CK), 1% (PN1), 2% (PN2), and 3% (PN3). The physical and chemical properties, N fraction concentrations, ammonia volatilization, and N loss rates were measured and explored during composting process. The results showed that the pile temperature of NPA treatments were higher than that of CK in thermophillic period, however their water contents were significantly (P＜0.05) lower than that in CK in cooling period. At the end of composting, the concentrations of total nitrogen and organic nitrogen increased significantly in NPA treatments (P＜0.05), and their highest concentrations in the PN3 treatment were 16.3% and 13.2% higher than those in CK, respectively. The ammonia volatilization losses of PN1, PN2 and PN3 treatments were 25.9%, 31.5% and 42.4% lower than that of CK, respectively, however, their nitrogen fixation rates reached 31.3%, 40.7% and 72.2% respectively. Therefore, adding NPA could accelerate start-up speed, shorten composting time, and also could effectively reduce ammonia volatilizations and nitrogen loss in the composting process of E. crassipes. Therefore, PN3 showed the best effects of nitrogen preserving.