A glass ceramic and a preparation method thereof include a step of melting calcined electrolytic manganese slag to obtain molten glass. The molten glass is sequentially molded, nucleated, and crystallized to obtain the glass ceramic. The calcined electrolytic manganese slag is taken as a raw material, so that harmful gases are not generated in the raw material melting process, and secondary pollution is not caused. The preparation method provided by the present invention has the advantages of simple operation, high utilization rate of electrolytic manganese slag and low cost. Moreover, the prepared glass ceramic has good mechanical properties and corrosion resistance.