The invention relates to an auxiliary material for casting a mould, in particular to a casting mould water-based coating formulation for a sand mould or an expendable pattern casting (EPC), and a process for preparing the same. The casting mould water-based coating formulation consists of refractory powder, a suspending agent, a bonding agent, a surfactant, a defoaming agent, a corrosion remover and water. The process comprises the following steps: adding the suspending agent to the water directly, and mixing and dispersing the mixture in a high speed dispersion machine; adding auxiliary agents including the defoaming agent, the surfactant, the corrosion remover, and the like to the water, and mixing the auxiliary agents; adding the refractory powder to the mixture to be mixed; and adding the bonding agent to the mixture to be mixed, and then adding water to adjust the coating until the coating has the thickness in a using state to obtain the finished product. The coating prepared by the process has the effects that the coating ensures that a high-density low-viscosity coating has excellent suspension property and storage stability, a coating layer is not easy to crack after drying, the coating is advantageous to improve the capacity of the coating to resist high temperature metal corrosion, and the like.