The invention belongs to the field of high polymer materials, and in particular relates to a directly coated polyolefin film and a preparation method thereof. The polyolefin film comprises a grafted layer of a certain thickness, the content of polar elements in the grafted layer is 20-35wt%, and moreover the polyolefin film has good surface properties. The invention further provides a preparation method of the polyolefin film. The preparation method comprises the following steps: putting the surface of the polyolefin film in a plasma reaction cavity, introducing gas polar molecules, adjusting the temperature and the pressure, performing low-temperature plasma treatment, and completing gas-phase grafting reaction. The directly coated polyolefin film prepared by using the preparation method is good in impressionability and adhesion solidness, and the surface polarity of the directly coated polyolefin film can be effectively maintained.