The invention relates to a polyethylene microporous film and a preparation method and an application thereof. The method comprises steps that the polyethylene resin is melt and blended with diluent toform fusant; the fusant is subjected to molding and cooling treatment to obtain a sheet, and phase separation of the fusant is achieved through cooling treatment; the sheet is subjected to longitudinal stretching treatment and first transverse stretching treatment to obtain a base film; the base film is subjected to extraction treatment to remove the diluent; the base film after diluent removal is dried to remove extractant; the base film after drying treatment is subjected to second transverse stretching treatment; the product after second transverse stretching treatment is subjected to three-stage transverse thermal relaxation treatment to obtain the polyethylene microporous film. The method is advantaged in that operation is simple, strong controllability is achieved, when the production speed of the polyethylene microporous film is 50-80 m/min, the transverse heat shrinkage rate can be as low as -0.5-0%, and the longitudinal heat shrinkage can be as low as 2-2.5%.