A preparation method of a loop heat pipe evaporator, being a hot-press sintering method comprising the steps as follows: putting a shell of the evaporator into a mould, then, uniformly and compactly filling corresponding positions in the mould with material powders of an evaporation core, a heat insulation core and a transmission core, applying a pressure high enough to tightly fit the evaporation core and the transmission core to the shell at corresponding sintering temperatures of powder materials for the evaporation core and the transmission core, carrying out hot-press sintering for molding, carrying out cooling after making the powder materials of the evaporation core and the transmission core form metallurgical bonding, and carrying out demolding to obtain the loop heat pipe evaporator; the mould being provided with corresponding structures shaped like steam channels on positions where the evaporation core is provided with the steam channels; the evaporator being composed of the shell and a composite capillary core; the composite capillary core being formed by sequentially compounding three layers including the evaporation core, the heat insulation core and the transmission core; the heat insulation core being located between the evaporation core and the transmission core; the side, not adjacent to the heat insulation core, of the evaporation core being provided with the steam channels, and the side, not adjacent to the heat insulation core, of the transmission core being close to a liquid storage device of a loop heat pipe; the evaporation core and the transmission core being made of the same material of which the heat conducting coefficient is larger than that of the material of the heat insulation core and the melting point is lower than that of the material of the heat insulation core; the melting point of the material of the shell being larger than or equal to that of the material of the evaporation core and the transmission core; all the evaporation core, the transmission core and the heat insulation core adopting the powder materials, the evaporation core and the transmission core being molded by hot-press sintering and tightly fitted to the wall surface of the shell to realize seal, and the heat insulation core which is sandwiched in the center being kept in a powdery state; and the particle size of the material of the transmission core being larger than or equal to that of the material of the evaporation core; and the particle size of the powder material adopted by the evaporation core is 48-13 μm (300-1000 meshes), the particle size of the powder material adopted by the transmission core is 297-48 μm (50-300 meshes), and the particle size of the powder material adopted by the heat insulation core is 297-48 μm (50-300 meshes); the particle size is measured based on meshes/sieving. the mould being provided with corresponding structures shaped like steam channels on positions where the evaporation core is provided with the steam channels; the evaporator being composed of the shell and a composite capillary core; the composite capillary core being formed by sequentially compounding three layers including the evaporation core, the heat insulation core and the transmission core; the heat insulation core being located between the evaporation core and the transmission core; the side, not adjacent to the heat insulation core, of the evaporation core being provided with the steam channels, and the side, not adjacent to the heat insulation core, of the transmission core being close to a liquid storage device of a loop heat pipe; the evaporation core and the transmission core being made of the same material of which the heat conducting coefficient is larger than that of the material of the heat insulation core and the melting point is lower than that of the material of the heat insulation core; the melting point of the material of the shell being larger than or equal to that of the material of the evaporation core and the transmission core; all the evaporation core, the transmission core and the heat insulation core adopting the powder materials, the evaporation core and the transmission core being molded by hot-press sintering and tightly fitted to the wall surface of the shell to realize seal, and the heat insulation core which is sandwiched in the center being kept in a powdery state; and the particle size of the material of the transmission core being larger than or equal to that of the material of the evaporation core; and the particle size of the powder material adopted by the evaporation core is 48-13 μm (300-1000 meshes), the particle size of the powder material adopted by the transmission core is 297-48 μm (50-300 meshes), and the particle size of the powder material adopted by the heat insulation core is 297-48 μm (50-300 meshes); the particle size is measured based on meshes/sieving.