The invention relates to composite light building block and preparation method thereof. The slurry comprises brine, light burnt powder, filler, air producing agent, foam stabilizer, adjusting agent, cement, water and aggregate. Taking a 60*15*25cm building block with the volume weight of 18kg as an example, the slurry is prepared from the following raw material in percentage by weight: 30-50% of brine, 0.008-0.01% of air producing agent, 25-35% of light burnt powder, 0.0005-0.001% of foam stabilizer, 20-30% of filler, and the balance of adjusting agent. The invention can improve the processing property, increases the slurry flowability, is easy to cast in forming, has short production period and low manufacturing cost, increases the activity, enhances the impermeability and post strength, reduces dry shrinkage, and prevents cracking. The composite light building block has the advantages of light weight, high strength, small volume density, low heat conduction coefficient, heat insulation, sound insulation, fire prevention, heat isolation, freezing and melting resistance, little dry shrinkage, and the like. By using the gas-added building block to replace the traditional bearing building block as the building wall filling material, the self weight of the building structure is decreased, the foundation burden is reduced, and the energy and material are saved.