The invention provides a hydrogen boiler based on coal gasification and water decomposition, including a steam boiler which includes an upper furnace and a lower furnace; water and steam in the upper furnace are respectively communicated with water and steam in the lower furnace; and the steam boiler is provided with a casing which has a narrow gap for containing water and a wide wall for heating to generate gas. The steam boiler contains multistage reactors. A coal modification and gasification device is provided at an outer side of the steam boiler and is provided with two chambers. The steam and gasified gas of coal are mixed and enter the reactors for direct burning to promote respective reactions. After several stages of modification and decomposition, the steam and the gasified gas of coal are completely converted to hydrogen.