The invention discloses a preparation method of composite solid biomass fuel. The preparation method comprises the following steps that reed straw is ground and then dried; fresh corn stalks are crushed; fresh pond sludge and the crushed fresh corn stalks are mixed to be uniform according to a certain volume ratio and then fermented to obtain a primary fermentation product; the primary fermentation product, the ground and dried reed straw and blue algae are sequentially and alternately laid layer by layer and then fermented to obtain a secondary fermentation product; the secondary fermentation product is compressed and cut into primary compressed materials; the primary compressed materials are cut into small pieces of secondary compressed materials; the surfaces of the secondary compressed materials are wet with blue alga water and then adhered with a layer of wood chips, the secondary compressed materials adhered with the wood chips are subjected to secondary extrusion and drying treatment, and after drying is finished, the finished solid biomass fuel product is obtained. According to the method, all kinds of biomass fuel with great difference in physical and chemical characteristics are reasonably mixed and compressed, and then the green solid biomass fuel which is suitable for combustion and high in combustion heat is prepared.