To achieve reliable heat-assisted magnetic recording (HAMR), it is crucial to understand the decomposition of perfluoropolyether (PFPE) lubricant films. In this study, we performed reactive all-atom molecular dynamics simulations to investigate the thermal decomposition of PFPE D-4OH using our original ReaxFF force field. The results showed that the thermal decomposition of D-4OH molecules is mainly attributed to the dissociation of C–O bond in the polar end groups and the dissociated fragments transfer from the disk to head and form smears. On the other hand, by quantifying the decomposition time constants at 700 and 800 K, we infer that the thermal decomposition of D-4OH films hardly occurs within the HAMR heating time of ~1 ns.