The invention belongs to the technical field of polymer films for electronics tapes and particularly relates to a preparation method of biaxially oriented polyester film for electronics tapes and biaxially oriented polyester film; the method includes the steps of pretreating materials, melt-extruding, stretching and drawing; an extruder used in the melt-extruding step has filtration precision of a head strainer being 5-10 Mum; the stretching step includes sequential steps of longitudinal stretching, transverse stretching and film shaping, wherein the film shaping step includes sequential steps of thermal shaping and quick cooling, a platinum catalyst and a discharge device are mounted in an area of film thermal shaping, and part of oligomers around the film are decomposed with decomposed products discharged; undecomposed oligomers of the film and oligomer dust are separated from the film by a suction device; the film prepared herein has very small thickness and further improved insulating property and heat resistance, and each part of the film is more uniform in performance.