A new method for modifying vermiculite with Na+ and K+ was proposed to improve its thermal expansion performance. High-temperature expansion experiment was used to measure the expansion rate of vermiculite before and after modification in various dimensions, and the microstructure and phase composition of various modified vermiculite were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The result of expansion experiment indicates that the expansion ratios of modified vermiculite increase with the increase of vermiculite particle size and experiment temperature with the temperature range of 100 ℃–900 ℃ when the particle size of vermiculite increases from 0.2 mm to 0.8 mm, and the modification effect of adding KCl is better than that of adding NaHCO3. The result of SEM shows that the interlayer spacing of Na-vermiculite is smaller than that of K-vermiculite after high-temperature expansion. The result of XRD shows that there are gold mica and mixed layer minerals of vermiculite and phlogopite in vermiculite, and inorganic salts do not damage the original layered structure and strength of vermiculite after modification.