The invention relates to a small-sized septic tank for treating domestic sewage of peasant households. The small-sized septic tank comprises an initial septic unit, an intermediate septic unit and a high-quality liquid manure storage unit which communicate with one another sequentially, wherein the initial septic unit is provided with a manure inlet pipeline, and the high-quality liquid manure storage unit is provided with a liquid manure outflow pipeline; fresh manure enters the initial septic unit from the manure inlet pipeline, the manure in the initial septic unit starts to be subjected to fermentation and decomposition, liquid manure is naturally divided into three layers, i.e., an upper layer, a middle layer and a lower layer due to different specific weights, the upper layer is pasty manure skin, the lower layer is lumpy or granular manure residues, and the middle layer is liquid manure; the liquid manure of the middle layer overflows to the intermediate septic unit through a pipeline, the liquid manure of the intermediate septic unit is subjected to further fermentation and decomposition, ova continue to sink, and pathogens gradually die; and the further detoxificated liquid manure overflows to the high-quality liquid manure storage unit through a pipeline. Compared with the prior art, the small-sized septic tank has the advantages of simple structure, good treatment effect, convenience in construction, convenience in maintenance, and the like.