The invention provides a coal ash foam building block production method. According to a formula, a dry blend is composed of coal ash, blast furnace slag, gypsum powder, perlite and cement, and polystyrene particles, water and various additives are added in each ton of dry blend. A production system is composed of hoppers, stock bins, a polystyrene particle puffing machine, electronic scales, an auger conveyor, a water pump, a stirring machine, mold trolleys, a light rail and a bin for the various additives. The production system is characterized in that the electronic scales which are controlled by a computer are installed at outlets of the stock bins correspondingly and are in butt joint with the auger conveyor, the auger conveyor is connected with the feeding end of the stirring machine, and an outlet of a polystyrene particle bin is connected with the stirring machine; the puffing machine is connected with a steam pipe; the stirring machine is connected with a water pump water outlet pipe; the mold trolleys are arranged on the light rail, and mold assemblies are arranged on the mold trolley and are in butt joint with an outlet of the stirring machine; and wet blends are poured into molds for formation, manual demolding is conducted, and water is poured for curing. According to the coal ash foam building block production method, waste can be changed into valuable materials, the formula is nontoxic and free of odor and radioactive contamination, and coal ash foam building blocks produced through the method are pure-natural, green, healthy, ecological and environment-friendly wall materials.