The invention discloses a monomer containing a benzimide phenylacetylene structure. The monomer can enable self-crosslinking chemical reaction of a synthesized polymer under high temperature. The monomer containing a benzimide phenylacetylene structure and the structural units represented by I and II for synthesis of polyester can undergo random copolymerization to obtain high temperature self-crosslinked copolyester, and when the copolyester contains a PET structural unit, the characteristic viscosity number [eta] is 0.44-1.38dL/g, the vertical combustion UL-94 grade is from V-2 to V-0, the oxygen index LOI is 24.0-35.0%, and the cone calorimeter test peak heat release rate PHRR and the smoke release total amount are both lower than pure polyester. The invention also discloses a preparation method of the copolyester. The benzimide and phenylacetylene in the copolyester involved in the invention can generate synergistic cross-linking effect, so that the copolyester has very high flame-retardant and anti-dripping efficiency, and also the copolyester is endowed with excellent flame retardant, anti-dripping and smoke suppressing performance. The method provided by the invention is basically consistent with the synthesis method of conventional polyester, not only is mature in technology, and easy and convenient to operate, but also is easy to control and convenient for industrial production.