Abstract: The technology of investment casting with fly ash as back layer powder, ethyl silicate as binder and coal gangue sand as sanding material was studied. The properties of coating viscosity, suspension, solidification time and shell strength were tested. The influence of powder-liquid ratio and catalyst on the properties of the coatings was obtained and the ratio of the coatings was optimized. The test proves that fly ash is feasible as the back layer material of investment casting.