The invention discloses a method for producing a flat filament by using PET (Polyethylene Terephthalate). The method is characterized by comprising the following steps: (1) drying PET particles, then, hot melting and blending the dried PET particles with a toughening agent, an anti-hydrolysing agent, an antioxidant and a nucleating agent in a screw extruder, extruding and forming a primary flat filament; and (2) carrying out water bath cooling, drafting for the first time, carrying out primary water bath, drafting for the second time, carrying out secondary water bath, drafting for the third time, carrying out drawing and heat shaping, cooling, and finally, rolling to obtain the PET flat filament. Compared with the prior art, the method has the advantages that a proper draw ratio is adopted in a PET melt extrusion process to ensure that the PET sufficiently reacts with auxiliaries, so that the melt strength and toughness of PET are improved, and the drafting performance of the flat filament is improved; and proper temperature, drawing speed and multiplying power as well as multiple heating and drafting are adopted, so that the PET drafting orientation is benefited, the proper PET crystallinity is controlled, the filament breakage is avoided, and the tensile strength of the produced PET flat filament is up to over 50MPa.