The invention relates to a foamed concrete and a preparation method thereof. The foamed concrete adopts a chemical foaming method, and a foam stabilizer, an early strength agent, and a water reducer are respectively added to ordinary Portland cement slurry and stirred evenly, and then chemical foaming agent is added. The foaming agent is fully stirred evenly, then injection molded, cured at room temperature, and hardened. The dry apparent density range of the foamed concrete prepared by this method is 150-300kg / m3, the corresponding compressive strength is 0.10-0.50MPa, and the thermal conductivity is lower than 0.10W / mK. It can be used as roof insulation project, floor heating project, light Cushion material, especially suitable for external wall insulation system, energy-saving composite panels, etc. used in building energy-saving projects.