The invention discloses an ultra-supercritical heat-resistant steel flux-cored wire and a preparation method thereof. The flux-cored wire comprises a skin and a flux core, and the skin is a low-carbon steel thin strip; by mass percentage, the flux core comprises 3-5% of graphite powder, 35-50% of metal chromium, 20-40% of rutile, 3-8% of ferromolybdenum, 3-8% of ferromanganese powder, 1-8% of nickel powder, 2-8% of feldspar, 1-5% of ferrovanadium, 1-5% of ferroniobium, 0.2-2% of fluoride and the balance iron powder and inevitable impurities; and the weight of the flux core accounts for 10-30% the total weight of the wire. The preparation method includes the steps that all components of the flux core are uniformly mixed according to the ratio and then, placed on the low-carbon thin steel strip as the skin, wrapped or coiled into filaments and made into the preset specification through drawing and die-down, and the ultra-supercritical heat-resistant steel flux-cored wire is obtained.