The invention discloses nanoscale zero-valent iron stable in air, wherein nanoscale zero-valent iron is taken as a core, and silicon dioxide is covered on the surface thereof. The preparation method comprises the following steps of: (1) adding isopropanol aqueous solution in ferrous sulphate, charging nitrogen, mechanically stirring for 15 to 30 minutes, dropping sodium borohydride solution containing sodium hydroxide while stirring, after that, adding polyethylene glycol, and continuing to react for 0.5 to 1 hours while stirring; (2) adding tetraethoxysilane in the solution, controlling the molar ratio of the sodium hydroxide to the sodium borohydride in the solution to be in a certain range, continuously stirring, and continuously charging nitrogen, after the reaction is finished, staying, vacuum-filtrating, then washing by absolute ethyl alcohol and deionized water respectively; and (3) vacuum-drying the obtained nano-iron to obtain the stable nanoscale zero-valent iron. The nanoscale zero-valent iron has a good inoxidizability in air; and the whole reaction process has the advantages of low material price, simple device, convenient operation, and easily-controlled and safe process condition.