The invention relates to a high-size special-shape carbon-based composite material member and a preparing method thereof. The member includes, by mass, 20-35% of carbon fibers, 35-75% of pyrolytic carbon and 5-15% of SiC. The preparing method includes needling the continuous carbon fibers into a prefabricated blank the density of which is 0.4-0.55 g/cm<3> according to a required shape of the member; then fixing the inner and outer surfaces of the prefabricated blank by adopting a graphite mold; and then performing high-temperature thermal treatment, a first time of carbon deposition, demolding, coarse machining, a second time of carbon deposition, graphite processing, silicon vapor bathing and finish machining in order to obtain a finished product. According to the member and the method, a problem that high-size special-shape carbon-based composite material members are liable to deform in long-time standing processes and preparing processes is overcome, and the obtained product has advantages of good mechanical performance, a low thermal expansion coefficient, high thermal conductivity, good chemical stability, and the like, is suitable for industrial production, and can be applied in the field of aviation and aerospace, the field of communication and transportation, the field of biomedicine, and the like.