Clay minerals of surface sediments in the salt lakes of Hol Xil area were studied in this paper with the purpose of obtaining some recent environmental information of the lakes. The results show that clay minerals are illite and chlorite in surface sedimentary samples, whereas non-clay minerals mainly include quartz, calcite and feldspar together with small amounts of dolomite, aragonite, amphibole and gypsum. Clay minerals with particle sizes smaller than 2 μm in samples are illite (65%～82%) and chlorite (13%～35%), and small amounts of ordered Illite/Smectite (I/S) mixed layer (4%～15%) also exists in local areas. According to the distribution of samples and the combination characteristics of clay minerals, the samples can be divided into 3 districts: mineral combinations of samples in No.1 district and No.3 district are illite, chlorite and ordered I/S mixed layer; however, samples in No.2 district are composed of the combination of illite and chlorite, with no I/S mixed layer. Illite crystallinity in samples was obtained by measuring full width half maximum (FWHM) of 10 Å diffraction peak according to Kübler Index. At the same time, standard curve of illite crystallinity was obtained by international standard illite samples under the condition of D/Max-RC equipment, and the linear equation is y=1.920 4 x-0.154 7 with related coefficient R²=0.982 8. On the basis of the linear equation for illite standard curve, the corrected illite crystallinity Kübler Index (IC) in samples was obtained, and the chemical index of illite in samples was also calculated. The crystallinity values for samples in No.1 and No.3 district are in the range of 0.41～0.59 with the average value of 0.51. However, illite in samples of No.2 district has better crystallinity because its crystallinity values are in the range of 0.25～0.40 with the average value of 0.34, obviously smaller than the values of No.1 and No.3 district. In addition, chemical indexes are all smaller than 0.5, showing that illite belongs to Fe-Mg illite resulting from physical weathering. In conclusion, illite and chlorite, as major clay minerals in samples, suggest that the environmental conditions of the studied lake area are arid and cold, whereas small amounts of ordered I/S mixed layers in local districts, i.e., No.1 and No.3 districts, indicate that these districts have undergone the environment of alternate transient arid and humid in recent years. Furthermore, under acidic medium conditions, small amounts of illite experienced weak hydrolysis which led to the loss of potassium ions from illite crystal lattice, and then transformed to ordered I/S mixed layer, accompanied by the decrease of the crystallinity of illite.