This paper studies the accumulation mechanism of different types of inorganic solids in activated sludge system by taking two main kinds of inorganic solids-chemical sludge and sediment as the research object. In the chemical phosphorus removal experiment, self-made calcium silicate hydrate was used as the chemical phosphorus removal agent and added to the end of the biochemical system of domestic sewage for phosphorus removal experiment, while the fine sediment was simulated by quartz sand of 26, 73, 106, 165 and 210 μm respectively, and a complete mixture model of inorganic solid accumulation was established as X_iss =C_{iss, inf} × (SRT/HRT + 1) × (1 – e^–t/SRT). The results showed that the self-made hydrated calcium silicate had good phosphorus removal effect, and the TP of domestic sewage could be removed from 7 mg/L to 0.5 mg/L. The chemical sludge accumulation in the phosphorus removal process conforms to the complete mixing model, and the relative errors of measured and predicted values of MLVSS/MLSS and MLISS were 1% and 3%, respectively. The sediment accumulation process does not conform to the completely mixed model. The relative error of MLVSS/MLSS is between 56% and 77%, and that of MLISS is between –45% and –80%. The accumulation forms of inorganic solids in different forms of activated sludge systems are also different. Chemical sludge is mainly suspended in the mixture, reducing the proportion of MLVSS/MLSS. In activated sludge system, sediment is accumulated in various forms such as suspension and deposition.