The invention relates to the field of polyethylene films and particularly relates to a polyethylene composition, a preparation method thereof, a polyethylene film and a preparation method thereof. The polyethylene composition contains a product prepared by the steps of carrying out first polymerization reaction by virtue of ethylene and alpha-olefin in the presence of a catalyst, and carrying out second polymerization reaction by virtue of a product of the first polymerization reaction, ethylene and alpha-olefin, wherein the weight-average molecular weight Mw of a product of the second polymerization reaction is 80,000g/mol-200,000g/mol due to the conditions of the first polymerization reaction and the second polymerization reaction, Mw/Mn is more than or equal to 4.0, Mz/Mw is more than or equal to 4.0, and Mz+1/Mw is more than or equal to 4.5. The polyethylene composition is high in stretching ratio and film forming speed and very good in film forming property when being used for preparing the polyethylene film, and furthermore, the polyethylene film prepared from the polyethylene composition further has relatively excellent mechanical properties and optical properties.