Low grade magnesite is one of the main research directions in the future as the raw material for the preparation of magnesiabased insulating refractories. Periclase-forsterite (MgO-Mg2SiO4) lightweight insulating refractories were prepared by the molten salt method with high silica magnesite and tertiary talc ore as raw materials by pretreating them to get light burnt magnesia and talc, and NaCl molten salt as the reaction medium. The effects of the NaCl addition, the sintering temperature, the holding time and the raw material ratio on the sample preparation were studied. The results show that when the NaCl addition is 20% of the mass of light burnt magnesia and talc mixture, the sintering temperature is 1 200 ℃, the holding time is 6 h, and m(light burnt magnesia) : m(talc)=5 : 5, the sample has the optimal comprehensive properties: the bulk density of 1.46 g·cm-3 and the apparent porosity of 55.0%. In addition, it is found that self-decomposition of talc and the formation of forsterite can form pores inside the sample.