The invention discloses a horizontal coal gas gasifier and a gasification method thereof, and relates to the field of clean coal gasification. According to the horizontal gas gasifier and the gasification method thereof, the horizontal gasifier comprises a gasification reaction chamber, a gas-slag separation section and a slag pool which are sequentially arranged from top to bottom, burner channels are symmetrically arranged on the two sides of the gasification reaction chamber, pulverized coal is introduced into the burner channels for incomplete combustion, and high-temperature coal gas and slag generated by gasification reaction enter the gas-slag separation section from a slag discharging opening; a membrane type water cooling wall is vertically arranged in the middle of the gas-slag separation section, low-temperature coal gas enters the gas-slag separation section from a chilling ring arranged on the upper portion of the membrane type water cooling wall, the temperature of the high-temperature coal gas is reduced to be below the ash melting point of slag, the slag is solidified into solid slag, the coal gas and the solid slag are subjected to gas-slag separation at the bottom, the slag falls into the slag pool, coal gas enters a subsequent system from the coal gas outlet in the upper portion, the horizontally-arranged gasification reaction chamber and the membrane type water cooling wall are adopted to play a baffling role, and then the overall height of the gasifier is greatly reduced.