The invention relates to a preparation method of a low-expansion wave-transmitting coating, belonging to the technical field of wave-transmitting materials. The volume expansion stability of the material is modified through epoxy linseed oil. The reason is that the epoxy linseed oil has a chemical cross-linking network structure, and epoxy molecules are largely fixed by chemical cross-linking points, therefore, the free volume pore size of the system is generally determined by the network density of the matrix. After kaolin is calcined, metakaolin particles caused by dehydration are easy to scatter in the epoxy matrix uniformly, an interface formed by calcined powdery particles and the metakaolin-epoxy matrix can limit motion capability of a molecular chain of the epoxy matrix, so that therandom motion conformation number of the molecular chain is reduced, the free volume pores are reduced, the free volume size is reduced, the free volume variation coefficient of the material is further reduced, as a result, the shrinkage performance of the composite material is lowered, and the material is more expandable and stable.