The invention discloses a preparation method for a lost foam casting coating. The lost foam casting coating comprises a common coating, carboxy methylated cellulose (CMC), white latex, quartz powder, lithium-based bentonite, an active agent and water. The preparation method comprises the following steps that S1, an appropriate amount of water is added to the common coating, and the mixture is put in a coating bucket to be stirred through a mixer for 40-90 min; S2, a Baume meter is used for measuring the proportion of the coating; S3, the CMC is soaked 24 hours in advance; S4, the white latex, the soaked CMC and an appropriate amount of water are poured into the coating bucket to be stirred evenly through a high-speed mixer; S5, the quartz powder is slowly added into the coating bucket, and the mixture is stirred evenly through the high-speed mixer; and S6, the lithium-based bentonite is added into the coating bucket, and the mixture is stirred through the high-speed mixer for 40-60 min. Compared with other coatings, according to the preparation method, the preparation process steps are simplified, the strength of the coating is effectively improved, and the defects such as scrap inclusions, slag inclusions and sand adhesion of a casting due to the fact that in the molten iron casting process, a coating layer of a foam model is liable to fall off to generate coating scraps are overcome.