The invention relates to an energy-saving self-heat-insulation ceramsite building block and its manufacturing method and forming mould. The energy-saving self-heat-insulation ceramsite building block is made of, by weight, 35-50% of ceramsite, 20-29% of cement, 15-30% of coal ash, 0.5-1% of fibers, 2-5% of foaming agent, 0.3-0.5% of water reducing agent, 0.1-0.3% of waterproof agent and 0.2-0.5% of catalyst, wherein the volume of the ceramsite accounts for more than 80% of that of the whole building block. The manufacturing method of the building block includes steps of material selecting, metering, stirring, pouring, vibrating, steaming, tube drawing for demoulding, cutting forming and stacking for rest. The forming mould of the building block can be used for once forming of multiple medium-sized rectangular ceramsite building blocks, and each building block can be conveniently cut into standard small ceramsite building blocks. The energy-saving self-heat-insulation ceramsite building block is high in ceramsite content and has the advantages of light weight, warm retention, heat insulation, sound insulation, sound adsorption, low water adsorption, fire resistance, freezing resistance, quake proofing, corrosion resistance and the like.