The invention belongs to a gasification ash water low-pressure flash heat energy utilizing device and a heat energy utilizing method thereof. The gasification ash water low-pressure flash heat energy utilizing device comprises a gasification furnace; a gasification furnace chilling chamber ash water drainage pipeline is connected with an inlet of a low-pressure flash tank through a first pressure reducing valve; a bottom liquid phase outlet of the low-pressure flash tank is connected with an inlet of a vacuum flash tank through a second pressure reducing valve; a bottom liquid phase outlet of the vacuum flash tank is connected with an inlet in the top of a setting tank; an ash slag discharge port is formed in the bottom of the setting tank; a liquid outlet in one side of the upper portion of the setting tank is connected with a liquid inlet in the upper portion of a gas extraction tower through the tube pass of a first pump and the tube pass of a first heat exchanger; a gas outlet in the top of the gas extraction tower is sequentially connected with the shell pass of the first heat exchanger and an inlet of a gas and liquid separator through a pipeline. The gasification ash water low-pressure flash heat energy utilizing device has the advantages that the structure is simple, design is reasonable, low-pressure flash steam has the deoxidizing, heat exchanging, vacuumizing and gas extracting effects at the same time, byproduct low-pressure steam of the whole device is reasonably and efficiently utilized, and the water quality of the whole ash water system is improved.