The invention relates to a carbonaceous slurry and a preparation method thereof. The preparation method comprises the following steps: uniformly mixing coal tar pitch, stearate, polyphosphoric acid, a surfactant, an additive and a complexing agent, carrying out heat treatment, and sequentially crushing and screening the above obtained material to obtain a screened material A with the granularity being less than 0.088 mm; uniformly mixing the screened material A, maleic acid, zinc oxide powder, copper powder and the complexing agent, carrying out heat treatment, and crushing and screening the obtained material to obtain a screened material B with the granularity being less than 0.088 mm; and uniformly mixing electrically calcined anthracite with the granularity being 0.088-0.5 mm, electrically calcined anthracite with the granularity being less than 0.088 mm, amorphous graphite with the granularity being 0.088-0.5 mm, amorphous graphite with the granularity being less than 0.088 mm, the screened material B, silicon carbide powder and thermosetting resin to obtain the carbonaceous slurry. The preparation method has the advantages of wide sources of raw materials, and low production cost, and the prepared carbonaceous slurry has the characteristics of simplicity in construction, small high-temperature shrinkage, and large brick lining bonding strength.