The invention relates to a method for preparing a composite material at high temperature and high pressure, in particular to a carbon-graphite hot isostatic pressing immersion silver method, which is to put a graphite crucible and silver coated graphite ingots filled in the graphite crucible into a hot isostatic press together for hot isostatic pressing; the graphite crucible for hot isostatic pressing immersion silver is divided into a protective graphite crucible and a graphite crucible filled with the silver coated graphite ingots for immersion silver, and the graphite crucible for immersion silver is arranged in the protective graphite crucible; the diameter of the graphite crucible filled with the silver coated graphite ingots for immersion silver is 1 to 5 percent larger than that of the filled silver ingots; and the graphite crucible on the uppermost layer is provided with a blind nut which is in threaded connection with the graphite crucible for immersion silver. The method immerses silver of carbon-graphite substrate through the hot isostatic pressing, improves the structure of the graphite crucible to improve hot isostatic pressing immersion silver quality and efficiency, and prepares the carbon-graphite substrate with the density between 1.70 and 1.82g/cm<3> into a carbon-graphite composite material immersed with silver, of which the density is between 2.80 and 3.50g/cm<3> through the hot isostatic pressing immersion silver; and the strength, heat conductivity, electric conductivity and wear-resisting property of the prepared material are obviously improved, so the material has new application prospect.