Using TG–FT-IR coupling method, direct evidences of the decomposition process of lithium hexafluorophosphate had been obtained. Studies showed that LiPF6 is stable under normal temperature when the content of water is very low. When heated, LiPF6 would react with the trace water in the air. HF and OPF3 would be released in the same time which could be detected from the FT-IR spectra collected in situ. The peak temperature of the decomposition of LiPF6 varied from 192.93°C (average heating rate 2.0°Cmin−1) to 223.83°C (average heating rate 20.0°Cmin−1). Based on the on-line FT-IR spectra, the characteristic absorption bands of PF6 − in the decomposed substances were detected for the first time which might come from the reaction of trace HF with PF5.