Provided are a high temperature-resistant composite nanofiltration membrane and a preparation method thereof. The high temperature-resistant composite nanofiltration membrane includes a base membrane and a polyamide membrane arranged on the base membrane; wherein the polyamide membrane is prepared from raw materials comprising: an amine, an inorganic salt, a silane additive, a polyacyl chloride, and an oil phase solvent; and the silane additive is at least one selected from the group consisting of 3-aminopropyltriethoxysilane, divinyltriaminopropyltrimethoxysilane, N-cyclohexyl-γ-aminopropyltrimethoxysilane, and trimethoxy[3-(phenylamino)propyl]silane.