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 process, 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; and the immersion silver process comprises: heating the graphite crucible and the silver coated graphite ingots at 100-200Pa in a baker to 300 DEG C; introducing Ar gas with 0.1 to 0.2MPa and fully melting silver by temperature rise; cutting off a power supply for heating, vacuumizing to 200-1,000Pa vaccum degree; and filling Ar gas, supplying power for heating, maintaining the temperature at 1,200 to 1,260 DEG C and the pressure at 80 to 90MPa for 1 to 3 hours, and cutting off the power supply and cooling with the baker. The process 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.