Abstract: Based on the process and principle of the test method for average degree of polymerization of ammonium polyphosphate by end grouptitration,discussion has been carried out from the exchange process,exchange products and titration process.Conclusion was made by comparing with the test result of nuclear magnetic resonance spectroscopy：the end-group titration is only suitable for testing average degree of polymerization of ammonium polyphosphate product with high water dissolvability and not suitable for products with bigger average degree of polymerization and low water dissolvability.