The present application provides an adjustable mandrel and a fixture. The adjustable mandrel comprises an outer-diameter adjusting assembly and a height adjusting assembly, wherein the external diameter and the height of the mandrel can be adjusted according to wheels in different specifications, the wheels in different specifications are clamped, and the radial positioning of the wheel and the axial positioning of the wheel during second-step processing are realized. Two adjusting systems supplement each other, so that the structure is novel, the concept is ingenious, the universality is high, the application range is wide, and the machining cost is reduced.