The invention relates to a pressurizing and decompressing process for a coal lock by utilizing byproduct carbon dioxide gas and a coaling system. The coal lock pressurizing and decompressing method comprises the following steps of opening an upper conical valve of the coal lock, and closing a lower conical valve; charging the feed coal into the coal lock through a coal hopper, and closing the upper conical valve after the feed coal is charged; compressing and supercharging the carbon dioxide from a subsequent purification step through a compressor, and pressurizing the coal lock by utilizing the carbon dioxide after the upper conical valve is closed; opening the lower conical valve of the coal lock after the coal lock is pressurized to the pressure of a gasification furnace, and filling the feed coal into the gasification furnace through a transitional cabin; and closing the lower conical valve of the coal lock after the feed coal enters the gasification furnace, decompressing the coal lock through a pressurizing/primary decompressing pipe orifice on the upper part of the coal lock for the first time, and decompressing the coal lock through a secondary decompressing pipe orifice for the second time. The coaling system comprises a coal hopper, a coal lock, a transitional cabin and a gasification furnace, wherein the coal lock comprises an upper conical valve, a coal lock main body, a lower conical valve, a pressurizing/primary decompressing pipe orifice and a second decompressing pipe orifice.