Some factors affecting soil-fixed ammonium, such as organic matter, total N, organic N, clay, CEC, ammonium content in soil solution, moisture and temperature, were studied. The results indicate that soil-fixed ammonium is positively related to<0.01 mm clay content, but not much related to<0.001 mm clay, organic matter, total N, organic N or CEC. Fitting of the Langmuir equation reveals that the maximum ammonium-fixation by river mud is 357.2 mgN/kg, quite close to 359 mgN/kg, and that no matter whether tidal mud or Zhinitian, their ammonium fixation rates show a trend of 30℃ > 20℃ and 30℃ > 40℃ .Besides Huangnitian, ammonium-fixation rates of Zhinitain, river mud and tidal mud all have a similar trend of long-term inundation > 8 times dry and wet alternation > long-term drought.