The invention relates to the technical field of modified plastics, in particular to a low-expansion, flame-retardant and photodiffusion PC (polycarbonate) composite material for an LED lamp tube and a preparation method of the PC composite material. The PC composite material comprises raw materials in parts by weight as follows: 80-100 parts of PC, 4-25 parts of an enhancing additive, 0.3-3 parts of a flame retardant, 0.2-1.5 parts of a photodiffusion agent and 0.2-1 part of an antioxidant, wherein the enhancing additive is one or a mixture of flat glass fibers, glass powder, wollastonite, white carbon black, BDP and RDP; the flame retardant is a mixture of phenyl silicone resin crystals and PC carrier PTFE (polytetrafluoroethylene) which are mixed in any mixing ratio. The process is simple, the prepared material has low expansion and flame retardant characteristics through PC enhancement and flame retardant modification, and meanwhile, the material has good light transmittance, haze and mechanical property and can be applied to the field of ultrathin lamp tubes with large sizes.