In order to reveal the yellowing mechanism of regenerated polyethylene terephthalate (rPET) for preparing rPET with improved chromaticity, waste PET was glycolyzed in ethylene glycol with catalysts zinc acetate (Zn(Ac)2), potassium carbonate (K2CO3) or titanium phosphate (Ti3(PO4)4), and rPET was then prepared using the glycolysis products as raw material through polycondensation stabilized with triphenyl phosphate (TPP) or trimethyl phosphate (TMP). The catalyzing activity of three catalysts were investigated based on conversion of waste PET and yield of dihydroxyethyl terephthalate. Nuclear magnetic resonance, thermal analysis and intrinsic viscosity were used for exploring chemical structure of rPET with various catalysts and stabilizers. As for the investigation of redox side reactions in rPET, chromaticity analysis, terminal carboxyl agent content testing, fluorescence spectrum and ultraviolet spectrum are employed. The effect of addition and increasing content of catalysts Zn(Ac)2, K2CO3 and Ti3(PO4)4, as well as stabilizers TPP and TMP on the above redox side reactions causing chromatic agents and terminal carboxyl agents were thoroughly discussed, as a result, the yellowing caused by side effects of rPET and its inhibition mechanism was thus proposed.