Phase diagram of MgCl2–LiCl–Na2SiO3–H2O (MLSH) system at 200°C was reported. The phase diagram was designed according to the hydrothermal system in which hectorite was formed. The solid phases were characterized by X-ray diffraction (XRD) and energy dispersive spectrometer (EDS). Lithium silicate, hectorite and karpinskite were observed as solid phases. The presence of hectorite in solid phases was decided by the molar ratio of MgCl2 and Na2SiO3 in the system. When the Mg/Si molar ratio exceeded a certain value, little hectorite was formed even though there was a large amount of LiCl in the system. The reason may be that it was difficult for Li+ to replace the Mg2+ in the framework. As a versatile nanomaterial, hectorite could be synthesized using concentrated natural brine as starting materials by hydrothermal method at 200°C. The phase diagram could be used as the theoretical principle for synthesis of hectorite and comprehensive utilization of concentrated natural brine.