The invention provides a settling method of solid substances in carboniferous system coal slurry water, and solves the problems that great influence is caused to production of a coal preparation plant due to poor carboniferous system coal slurry water treatment effect. The settling method is characterized in that flocculant and polycoagulant are added in the carboniferous system coal slurry water, the flocculant adopts polyacrylamide with the molecular weight of more than 23 million and the degree of hydrolysis of 20% to 27%, the polycoagulant adopts polyaluminum cloride, the use level of the polyacrylamide is 3 to 4.4g/m<3>, the use level of the aluminium polychlorid is 40g/m<3>, the configuration concentration of the polyacrylamide is 100 ppm, and the configuration concentration of the polyaluminum cloride is 1,000 ppm. According to the settling method, the use level of the flocculant is greatly reduced, so that the thickness of the clear water layer in a thickener is larger than 1.5 m, the processing capacity of a pressurizing filter is increased by about 33%, the cost of the flocculant can be saved by 912,200 yuan every year, not only is the drug cost saved, but also the processing capacity of the pressurizing filter is increased, and the moisture of a filter cake is reduced.