The invention discloses a heat radiator with a composite structure, which relates to the field of electronic products heat radiators. A plurality of long fins made from expandable graphite composite materials and a plurality of short fins made from flexible graphite materials are staggered in a clamping device, wherein the long fins align with one ends of the short fins, the aligned end face is provided with a heat-conducting layer which is made of flexible graphite material or metal materials and has the thickness of 0.15-0.2 mm. The invention fully takes the advantage of the high heat conductivity of the flexible graphite plate material in the plane direction, optimizes a heat-conducting structure when the high heat conductivity of the flexible graphite plate material and the high heat-tolerance performance of the expandable graphite composite material are matched, forms a favorable three-dimensional heat dissipation effect and has light weight, little heat resistance, favorable contact, controllable temperature, short production cycle and low production cost. The invention also relates to a preparation method of the heat radiator.