The invention discloses a diameter-adjustable flexible core rod with a novel structure. The complete flexible core rod is formed by axial series connection of a plurality of inner sleeved core rod subsections through elastic rubber rods, the elastic rubber rods are arranged at through holes in the inner sleeved core rod subsections in a penetrating mode to enable the inner sleeved core rod subsections of a plurality of core rod sections to be elastically connected in series, and outer embedded core rod subsections are mutually nested on the outer portions of the inner sleeved core rod subsections to form the integral single core rod sections; and the multiple core rod sections are axially connected in series through the elastic rubber rods to form the complete flexible core rod, and the elastic rubber rods are arranged at the through holes in the inner sleeved core rod subsections/the outer embedded core rod subsections in a penetrating mode to conduct elastic series connection. According to the diameter-adjustable flexible core rod, the structure is simple, machining and assembling are easy, the core rod sections with the diameters being different can be formed through mutual nesting and connecting of the inner sleeved core rod subsections and the outer embedded core rod subsections, the function that the diameter of the flexible core rod can be adjusted is achieved, the diameter-adjustable flexible core rod can be suitable for bent pipe fittings with different pipe diameters, the flexible adjustability is achieved, materials can be saved, and the cost is reduced.