The invention relates to the field of phosphate solution formulas. The phosphate solution is quite suitable for the parkerizing process performed on the ultra-high-strength steel sheet material blank before hot stamping; a composite phosphate solution is used for performing surface treatment on the ultra-high-strength steel sheet material blank, so as to obtain a high-temperature resistant composite lithium iron phosphate-calcium sulphate film which is used for improving the service life of molds. A composite lithium iron phosphate-calcium sulphate surface treatment liquid which adapts to hot stamping of the ultra-high-strength steel sheet material blank is prepared according to the chemical composition ratio provided by the invention, and the preparation method is as follows: firstly, synthesizing lithium iron phosphate using iron oxide Fe2O3, lithium hydroxide LiOH.H2O, lithium hydroxide LiOH.H2O, ammonium dihydrogen phosphate (NH4)2HPO4 and a carbon source C; then, mixing calcium sulfate CaSO4 ultra-fine powder, lithium iron phosphate LiFePO4 ultra-fine powder, phosphoric acid H3PO4 and distilled water H2O according to a mass ratio of calcium sulfate CaSO4 ultra-fine powder, lithium iron phosphate LiFePO4 ultra-fine powder, phosphoric acid H3PO4 to distilled water H2O being (40-60):(10-30):(5-10):(808-912) to prepare a phosphating solution; placing the to-be-treated ultra-high-strength steel sheet material blank into the phosphating solution at 55-60 DEG C, so that the blank can be coated with a dark gray phosphating film; finally, carrying out heating dehydrogenation process on the blank, so as to reduce the harm caused by hydrogen embrittlement; carrying out the saponification process to further improve the lubrication between the hot stamped sheet material and the mold.