The invention relates to a method for preparing linear low-density polyethylene, which comprises the steps of: premixing polyethylene, propylene and 1-butylene according to the pressure ratio, adding n-hexane, triethylaluminum and a catalyst into a high-pressure reaction kettle, and using acidified ethanol containing 10 mass percent of HCl to terminate a reaction, wherein the reaction pressure is0.8 MPa; and the temperature is 80 DEG C. A carrier of the catalyst is SiO2, active substances are Ti and Cr, and the catalyst is added with an organic silicon compound electron donor, wherein the molar ratio of CrO3 to the SiO2 is equal to 0.1-20; the molar ratio of TiCl4 to the SiO2 is equal to 0.1-180; the molar ratio of the SiO2 to an organic silicon compound is equal to 400-10,000; and the molar ratio of Al to (the Ti+the Cr) is equal to 10-1000. The mist degree of the polyethylene is 11 percent at lowest, the molecular weight distribution index is between 6 and 40, the flow rate of a melt is between 0.2 and 20 g/10min, and the density is between 0.910 and 0.940 g/cm<3>.