The invention discloses a method for preparing coal water slurry from low-rank coal. The method is characterized by comprising the following steps: step 1: conveying the low-rank coal into a crusher to be crushed until the granularity is smaller than or equal to 5mm; conveying the crushed fine coal grains into a spiral extruding device; step 2: shearing and extruding the fine coal grains which are conveyed into the spiral extruding device under the pressure of 5 to 40MPa; after damaging inner pores in the fine coal grains, separating moisture in the fine coal grains to obtain wet sticky coal, wherein the separated moisture accounts for 10 percent to 60 percent of the total moisture; and step 3: conveying the wet sticky coal obtained by the step 2 into a mill and simultaneously adding an additive and water, and grinding the mixture into the coal water slurry with the concentration of 53 percent to 65 percent. The method for preparing the coal water slurry from the low-rank coal is extremely simple; waste heat is used for pre-heating brown coal and the brown coal is re-compacted, so that the pores of the low-rank coal can be effectively damaged and the surface properties of the low-rank coal are changed; the method has the advantages of low energy consumption, high efficiency and high slurry preparation concentration, and can be widely applied to preparation of the high-concentration coal water slurry by all types of the low-rank coal.