The thermostability of montmorillonite were studied by variable temperature infrared spectrum ( including variable temperature one-dimensional spectrum and variable temperature second derivative spectrum ) in the temperature range from 303 K to 393 K. It was found that the five modes resulting from montmorillonite infrared spectrum include νOH、νH-O-H、 νSi-O、 δOH and νO-Si-O. With the increase of temperature, the frequency, strength and peak type of infrared corresponding to the main functional groups of montmorillonite change obviously. The study demonstrates the key roles of variable temperature infrared spectrum in the analysis of thermostability of montmorillonite.