The invention discloses a preparation method for a foamed cement composite board. The method includes the steps of: welding a light steel frame, laying a glass fiber grid cloth, preparing a cement mortar lower surface layer, welding a steel truss, preparing a foamed cement layer, laying a glass fiber grid cloth, preparing a cement mortar upper surface layer, and conducting maintenance. The preparation method provided by the invention adopts a multi-layer composite foaming technology, the prepared foamed cement has independent and closed foam holes, uniform and controllable size, and has the advantages of light weight, high strength, thermal conductivity and low water absorption, greatly improve the performance of foamed cement composite board; using cement foaming layer of intermediate density is small, low heat conductivity coefficient, low water absorption and the like, thus greatly enhancing the performance of the foamed cement composite board. The middle of the foamed cement layer adopts foamed cement with small density and low heat conductivity coefficient, thus meeting the thermal insulation requirements of the composite board. The area close to the surface layer adopts foamed cement with large density and high strength, thus greatly improving the strength of the composite board. Integration of load-bearing and thermal insulation is realized by the foamed cement composite board. And by utilizing the chemical defoaming diffusion technology, the adhesion performance of the foamed cement layer and the surface layer are greatly improved.