NOVELTY - Preparing polyester film for photovoltaic cell backboard, comprises: (a) synthesizing polyester slices; (b) preparing mother particles; and (c) preparing film by biaxial stretching.    USE - The method is useful for preparing polyester film for photovoltaic cell backboard (claimed).    ADVANTAGE - The method of the invention can change active terminal carboxyl groups on two ends of polyester macromolecule to stable groups; therefore, the ageing resistance is good.    DETAILED DESCRIPTION - Preparing polyester film for photovoltaic cell backboard, comprises: (a) synthesizing polyester slices: first, adding terephthalic acid, ethylene glycol and catalyst antimony acetate to a reaction container according to a certain proportion; increasing the temperature to 265 plus minus 5 degrees C to carry out esterification reaction; preparing heat stabilizer triethyl phosphate and antioxidant 1010 by ethylene glycol; adding triethyl phosphate and antioxidant 1010 prepared by ethylene glycol when the esterification rate of the esterification reaction reaches 90-95%; and reacting for 3-5 hours at 280-290 degrees C under the pressure of 25 plus minus 2 Pa to discharge to obtain polyester slices with viscosities of 0.68-0.72 and melting points of 258-260 degrees C, where the charging weight pts.wt. of the raw materials are as follows: 86-88 pts.wt. of terephthalic acid, 31-33 pts.wt. of ethylene glycol, 0.03-0.06 pts.wt. of antimony acetate, 0.05-0.10 pts.wt. of triethyl phosphate and 0.05-0.10 pts.wt. of antioxidant 1010; (b) preparing mother particles: crushing the polyester slices synthesized in the step 1 by a crusher; drying the polyester slices in a vacuum drum for 3 plus minus 0.5 hours at 80 plus minus 10 degrees C; after adding polycarbodiimide, UV absorbent MV-3638, triethyl phosphate and antioxidant 1010 to be mixed, extruding out to granulate by a double-screw extruder, where the charging weight pts.wt. of the raw materials are as follows: 85-98 pts.wt. of polyester slices, 1-15 pts.wt. of polycarbodiimide, 1-5 pts.wt. of UV absorbent MV-3638, 0.1-0.5 pts.wt. of triethyl phosphate and 0.1-0.5 pts.wt. of antioxidant 1010; the temperature of each area of the double-screw extruder is as follows: the temperature of the area I is 70-120 degrees C, the temperature of the area II is 130-200 degrees C, the temperature of the area III is 70-120 degrees C is 200-250 degrees C, the temperature of the area IV is 200-250 degrees C, the temperature of the area V is 200-250 degrees C, and the temperature of machine head is 200-250 degrees C; and the staying time of the material in the double screws is 3.5 plus minus 1 minutes; and (c) preparing film by biaxial stretching: after placing 62-89 pts.wt. of polyester slices synthesized in (a), 10-30 pts.wt. of mother particles prepared in (b) and 1-8 pts.wt. of PET polyester slices containing 3000 plus minus 100 ppm silicon oxide (SiO2) by weight in the double-screw extruder to be blended, drying the mixture for 10-60 minutes at 160-180 degrees C; and then placing the mixture in the double-screw extruder to be melted and extruded; entering a piece casting machine to cast pieces; and carrying out biaxial stretching to prepare the film, where the temperature of each area of the double-screw extruder is as follows: the temperature of the area I is 150-180 degrees C, the temperatures of the rest areas are 250-280 degrees C and the temperature of the mould head is 265-290 degrees C; the longitudinal tension ratio is 2-5 times; the transverse tension ratio is 2-5 times; the film producing speed is 120-180 m/minute; finally, 100-200 mu m biaxially oriented polyester film is obtained.