The invention discloses a method for preparing a photoelectric composite submarine cable and belongs to the field of preparation methods of cables. The method comprises the process steps of drawing wire, stranding, co-extruding, wrapping, degassing, adding a lead sheath, extruding a polyethylene (PE) sheath, vertical cabling and stranding, stranding optical units, armoring steel wire, sleeving outer sheath, rewinding a finished product and the like. The processes of conductors, insulating wire cores, degassing, pressing lead, extruding the sheath layer, three-core vertical cabling, wrapping, armoring steel wire and the like are connected in series, a joint-free whole large-length three-core photoelectric composite submarine cable is realized. The flexible joint of the large-length three-core photoelectric composite submarine cable is effectively avoided, so that the normal use and the service life of the submarine cable are guaranteed.