Ammonium dinitramide (ADN) is a halogen-free green oxidizer, which is widely used in energetic materials because of its positive oxygen balance and low characteristic signal. However, ADN is easy to absorb moisture, which affects its storage and safety performances. It is of great significance to study the effect of water on its thermal decomposition mechanism. In this paper, the thermal decomposition processes of the ADN with water contents of 0 %, 1 %, 2 %, 5 %, and 10 % are simulated by large-scale reactive molecular dynamics simulations. The reaction pathway and the evolution of the main products are analyzed in detail. The results show that the ADN is not stable and decompose easily, namely ADN -> NH3(NH4)+HN3O4(N3O4). The H2O has a significant effect on the thermal decomposition of the ADN. The water contents of 2 % and 5 % can better promote the decomposition of the ADN and the degree of decomposition is also more thorough.