The invention relates to a prefabticated member of a magnesia-hercynite kiln mouth. The prefabticated member of the magnesia-hercynite kiln mouth is prepared from, by weight, 15-25 parts of electric smelting magnesium-aluminate spinel particles with the size of 5-8 mm, 5-20 parts of electric smelting magnesium-aluminate spinel particles with the size of 3-5 mm, 5-15 parts of electric smelting magnesium-aluminate spinel particles with the size of 1-3 mm, 5-15 parts of electric smelting magnesium-aluminate spinel particles with the size of 0-1 mm, 10-20 parts of electric smelting hercynite particles with the size of 5-8 mm, 5-15 parts of electric smelting hercynite particles with the size of 3-5 mm, 5-10 parts of electric smelting hercynite particles with the size of 1-3 mm, 5-10 parts of electric smelting hercynite particles with the size of 0-1 mm, 3-5 parts of zirconium carbonate with the size of 0-1 mm, 3-5 parts of silicon carbide with the size of 0-1 mm, 2-3 parts of silica powder screened with a 200-mesh screen and 2-8 parts of alumina micro powder. According to the prefabticated member of a magnesia-hercynite kiln mouth, by adding high-grade raw materials such as magnesium-aluminate spinel and hercynite in the formula, the product has the good high temperature resistance, alkali resistance, thermal shock resistance and the like; by adjusting size composition, an automatic flowing mode of the product is achieved, and the construction problem brought by alkaline pouring materials is avoided.