The invention discloses a method for inhibiting expansion of fluidized bed combustion coal ash/slag products, and relates to a method for preparing building materials by autoclaved cure. The method has the following steps: taking fluidized bed combustion coal ash/slag, clinker and cement as raw materials of a cementing material, grinding the granular and lumpish raw materials, adding sand aggregate and mixing water for stirring and forming, and then performing the autoclaved cure to prepare finished products. The method is simple, can adopt a conventional still kettle for production, has simple and convenient operation, low cost, the utilization rate of the fluidized bed combustion coal ash/slag reaching between 75 and 100 percent, and good effect of inhibiting the expansion of the fluidized bed combustion coal ash/slag products, contributes to environmental protection, obviously improves early strength of the fluidized bed combustion coal ash/slag products, contributes to promotion and application and the like. The method can be widely applied to preparation of fluidized bed combustion coal ash/slag building material products with stable volumes.