As a slow release and long acting fertilizer rich in nitrogen and phosphorus，ammonium polyphosphate has a good chelating ability to trace elements，which can effectively prevent metal ions in soil from being fixed，and thus be better used by plants.Titration analysis was used to determine the chelating ability of water soluble ammonium polyphosphate with differ ent degree of polymerization on magnesium ion（Mg2+），and the chelating rule of water soluble ammonium polyphosphate to medium element Mg2+ was studied，which laid a foundation for the preparation of chelate fertilizer.The results showed that the higher the degree of polymerization of water soluble ammonium polyphosphate was，the stronger was the chelating ability to Mg2+.The chelating ability of Mg2+ decreased first and then increased with the increase of pH，and gradually decreased with the increase of temperature.Under the conditions of pH=6.0 and 5 ℃，the chelating amount of ammonium polyphosphate with higher polymerization to Mg2+ reached 8.2 g（per 100 g APP）.Under the conditions of pH=8.0 and 5 ℃，the chelating amount of ammonium polyphosphate with lower polymerization to Mg2+ reached 5.6 g（per 100 g APP）.The chelating reaction was verified by Fourier transform infrared spectrometer（FT-IR）.