The invention discloses castable taking magnesium oxychloride cement as a binding agent and a preparing method thereof, and aims to fully use waste resources and develop low cement. The castable is prepared from medium-grade sintered magnesia with the particle size of 3-5 mm, medium-grade sintered magnesia with the particle size of 1-3 mm, medium-grade sintered magnesia with the particle size of 0-1 mm, 200-mesh medium-grade sintered magnesia and the magnesium oxychloride cement binding agent, wherein the magnesium oxychloride cement binding agent is prepared from light calcined magnesia powder, magnesium chloride, a mixed material and water. The preparing method of the castable comprises the steps that firstly, the light calcined magnesia powder and the mixed material are put into a stirrer, then clear liquid of a magnesium chloride solution is taken and added into the stirrer, and even magnesium oxychloride cement paste is formed in the stirrer through stirring; the medium-grade sintered magnesia is added, even stirring is performed, and the magnesium oxychloride cement castable is prepared through vibration forming. According to the castable, by adopting the magnesium oxychloride cement binding agent, the magnesium oxychloride cement castable has the advantage of resisting high temperature, and the refractoriness under load of the castable can reach 1600 DEG C or above.