The retention and release behavior of NH4HSO4 in fly ash cement composite system(composite system for short) and the influence of NH4HSO4 on the composite system was studied by means of simulation. The results show that NH4HSO4 in denitration fly ash decomposes in the composite system and releases ammonia, part of which remains in the composite system. When 30% denitration fly ash is added into cement, the remaining NH4HSO4has little effect on the pH value of the composite system, and it is concentrated in the first 1h. The influence of retained NH4HSO4 on cement hydration is also concentrated in the early stage, which reduces the hydration exothermic peak and delays the appearance of exothermic peak. With the continuous hydration of cement in the late stage, it has no effect on the hydration heat and hydration products of cement. With the increase of ammonia content, the release amount and content of ammonia in the composite system increase, and the retention ratio of NH+4 and NH3·H2O also increases; the release of ammonia leads to the increase of porosity and the number of macropores after hardening of the composite system; the increase of ammonia content in denitration fly ash, paste mixing and water cement ratio of the composite system will accelerate the release of ammonia.