Method for manufacturing polarizing film and polarizing film manufactured thereby

The invention provides a method for manufacturing a polarizing film, which comprises the following steps: tensioning a first protective film with a first tensile force; tensioning a second protective film with a second tensile force; adhering the first protective film and the second protective film to two sides of a substrate film respectively; tensioning a third protective film with a third tensile force; adhering the third protective film to the other side of the first protective film opposite to the other side of the substrate film to form a composite film; and tensioning the fourth tensile force to form the polarizing film, wherein the ratio of the arithmetic product of the first tensile force and the third tensile force to the arithmetic product of the second tensile force and the fourth tensile is between 0.015 and 8. The invention also provides a polarizing film manufactured by the method, which comprises the substrate film, the first protective film, the second protective film and the third protective film. The method for manufacturing the polarizing film can reduce the occurrence of the warping of the polarizing film.