A polyethylene terephthalate (PET) copolyester and a manufacturing method thereof are provided. The manufacturing method includes the following steps. A dispersion slurry including a talcum powder is formulated, wherein the talcum powder has an average particle size of 1 μm to 50 μm and an average specific surface area of 7 m2/g to 20 m2/g. Next, a terephthalic acid and an ethylene glycol are mixed, and the dispersion slurry and a crystallization accelerator are introduced, so as to carry out a transesterification reaction and therefore form a bis-2-hydroxylethyl terephthalate (BHET). Afterwards, a prepolymerization reaction and a polycondensation reaction are carried out to form a polyethylene terephthalate (PET) copolyester.