The invention relates to a polyester film for flexible or thin-film solar cells and a preparation method thereof. The film is prepared from the following components in parts by weight: 100 parts of polyester resin and 0.0001-0.1 part of nano aluminum powder. The preparation method comprises the following steps: blending and extruding the polyester, nano aluminum powder and the like by a screw blending and extrusion apparatus, filtering, introducing into a running die head, drying, cooling, drawing, coiling and the like to obtain the polyester film for solar cells. The product is attached to the photic surface of the thin-film solar cell or used as an upper packaging film of the solar cell to directly enhance the generating efficiency of the flexible or thin-film solar cell.