The invention discloses a flame-retardant PVC cable tube and a preparation method thereof. The flame retardant PVC cable tube comprises the following components: polyvinyl chloride resin, phenolic resin, flame retardant masterbatch, a plasticizer, an antioxidant, oxidized polyethylene wax, a filler, a coupling agent, a corrosion resistant filler, a shock resistance modifier and pigment. The corrosion resistant filler includes, by weight, 5-12 parts of modified aramid fibers, 16-20 parts of ceramsite, 3-8 parts of graphene, 15-30 parts of epoxy resin emulsion, 5-10 parts bentonite and 3-6 partsof a ceramic adhesive. The shock resistance modifier includes, by weight, 11-16 parts of polylactic acid, 7-13 parts lignin fiber, 4-8 parts beet pulp, 3-7 parts of soybean meal, 8-14 parts of polyolefin elastomer, 5-10 parts of piperazine pyrophosphate and 20-30 parts of polystyrene resin. The flame-retardant PVC cable tube has the advantages of good flame retardancy, high corrosion resistance and strong impact resistance.