The invention provides a high-temperature-resistant neutron and Upsilon-ray shielding composite and a preparation method thereof. The high-temperature-resistant neutron and Upsilon-ray shielding composite is made from, by weight, 20-200 parts of a base material, 100-800 parts of an anti-radiation functional aid, 5-100 parts of a fiber reinforced material, 50-150 parts of a flame-retardant functional filling material, and 1-10 parts of a processing aid. The high-temperature-resistant neutron and Upsilon-ray shielding composite prepared herein has good neutron and Upsilon-ray shielding performance and good flame retardance, may operate normally in the temperature range of 150-260 DEG C, has excellent resistance to high temperature, and has low average linear expansion coefficient; it is indicated that the material has good size stability, and stability of neutron and Upsilon-ray shielding performance can be guaranteed; the high-temperature-resistant neutron and Upsilon-ray shielding composite is suitable for use as a radiation-protective material in a high-temperature environment.