The invention discloses molding sand for casting ion balls, wherein the molding sand is prepared by the following raw materials in parts by weight: 300-320 parts of quartz sand, 100-120 parts of argil, 15-20 parts of iron oxide red powder, 30-40 parts of silicon carbide, 4-6 parts of caustic soda, 2-3 parts of sulfur powder, 50-60 parts of gypsum, 3-4 parts of zinc stearate, 1-2 parts of tripolycyanamide, 2-4 parts of carbamide, 10-12 parts of flour and an appropriate amount of water. The quartz sand and the argil are used in the molding sand; after calcining, the molding sand is rich in porosity, good in breathability, capable of preventing the iron balls from forming subsurface porosity and good in flowability; by grinding in the combination of zinc stearate, silicon carbide and the like, the molding sand is excellent in cohesiveness, plasticity, thermal stability and high in collapsibility; by using the silicon carbide, the molding sand is excellent in heat-conducting property; the casting defects caused due to the local overheating expansion are not generated.