The invention discloses coated core sand for casting a train steel casting and a preparation method of the coated core sand. The coated core sand is prepared from, by weight, 170-180 parts of hill sand, 30-40 parts of zircon sand, 20-30 parts of emery, 15-20 parts of ceramsite, 15-20 parts of montmorillonite, 2-4 parts of phenolic resin, 2-3 parts of BT resin, 1-2 parts of diethoxy-methyl-[3-(oxiran-2-ylmethoxy)propyl]silane, 0.6-0.8 part of urotropine, 0.5-0.7 part of rosin pentaerythrityl ester, 5-7 parts of tephra, 6-8 parts of quartz crucible waste, 2-3 parts of salty mud, 0.3-0.5 part of paratoluensulfonyl chloride, 0.2-0.4 part of (H2NCH2C6H4)2O , 1-2 parts of trioctyl trimellitate, 3-5 parts of bismuth oxide, 4-7 parts of ferronickel slag and 20-30 parts of ethyl alcohol. The coated core sand has very high sand burning resisting capacity and veining resisting capacity, has the beneficial effects of being high in strength, small in gas generating amount, good in heat resistance and the like and is suitable for producing the steel casting, and the defects such as veining, sand burning and pores can be remarkably reduced.