The invention discloses a resin self-hardening core sand for casting train iron castings and a preparing method of the resin self-hardening core sand. The resin self-hardening core sand is prepared from, by weight, 160-180 parts of sea sand, 20-30 parts of carborundum, 16-20 parts of refiring magnesia, 10-14 parts of paper clay, 6-8 parts of chalk soil, 10-15 parts of furan resin, 12-14 parts of cashew nut shell oil modified phenolic resin, 4-6 parts of polyimide, 3-5 parts of rare earth tailing, 5-7 parts of pandermite, 8-10 parts of waste foam glass, 3-5 parts of ferro silicon nitride, 1-2 parts of N-(beta-aminoethyl)-gamma-aminopropyl triethoxy silane, 2-4 parts of p-toluenesulfonic acid, 3-5 parts of phosphoric acid, 4-6 parts of castor oil maleic anhydride acid esters, 8-10 parts of triphenylmethane triisocyanate and 2-4 parts of styrene. According to the furan resin self-hardening sand, mixed resin is adopted, the method of adding single resin is changed, mutual effects between different kinds of resin including the furan resin, the cashew nut shell oil modified phenolic resin, the polyimide and the like are adopted for enabling the core sand to have the beneficial effects of being high in strength, small in gas-forming amount, low in thermal expansivity, good in breathability, good in collapsibility and the like, and the produced castings are high in degree of precision and good in surface quality.