The influence of pentaerythritol tetrakys 3-(3,5-ditert-butyl-4-hydroxyphenyl) propionate (1010) and octylated/butylated diphenylamine (L57) on the thermal–oxidative degradation of di-2-ethylhexyl sebacate (DEHS) was studied by using model-free thermo analysis under air atmosphere. Activation energies about the thermal–oxidative degradation of lubricants were evaluated by Kissinger–Akahira–Sunose (KAS) method. Pressure drop test (PDT) and pressurized differential scanning calorimetry (PDSC) tests were also employed to study their antioxidant performance in DEHS. The results showed that the addition of 1010 and L57 could enhance the oxidation stability of DEHS and increase the activation energy of thermal–oxidative degradation especially at the initial stage of thermal–oxidative decomposition. Meanwhile, a good corresponding relationship was found between model-free method and traditional test methods.