The present invention relates to a process for preparing a coal-water slurry comprising the steps of screening coal into a coarse-grained coal and a fine-grained coal; subjecting said coarse-grained coal to gravity classification to classify into a low ash coal, a middle ash coal and a high ash coal; wet grinding a mixture of the fine-grained coal, the middle ash coal and part of the low ash coal, and subjecting same to flotation; preparing a first slurry having a solid concentration of 40-60 wt. % from the low ash froth resultant from flotation; and mixing another part of the low ash coal with this first slurry and wet grinding this mixture. This process can accord the solid concentration of the product slurry with an optional target value by adjusting the amount of the low ash coal mixed with the first slurry, and can use the remaining low ash coal not to be added to the first slurry for the purpose of preparing the second product slurry.