A gasification furnace, comprising: an outer shell having an outer shell inlet formed at a top of the outer shell and an outer shell outlet formed at a bottom of the outer shell; an inner shell, which is disposed in and spaced apart from the outer shell, defines a gasification chamber therein, has an inner shell inlet corresponding to the outer shell inlet and formed at a top of the inner shell, and an inner shell outlet corresponding to the outer shell outlet formed at a bottom of the inner shell, and is fabricated by a membrane wall having a cooling water inlet and a cooling water outlet; a nozzle disposed at the tops of the outer shell and the inner shell so as to extend into the gasification chamber through the outer shell inlet and the inner shell inlet; a lower shell connected with a lower portion of the outer shell, defining a slag exhausting chamber therein, and having a slag exhausting port formed at a bottom of the lower shell and a gas discharging port formed in an upper portion of a side wall of the lower shell, wherein the gasification chamber is communicated with the slag exhausting chamber via the outer shell outlet and the inner shell outlet; a cooler connected with an outer bottom wall of the outer shell around the outer shell outlet, and having a cooling passage formed therein, a cooler water inlet, and a cooler water outlet; a positioning member disposed between the inner shell and an inner bottom wall of the outer shell; and a gas guiding pipe defining an upper end connected with the cooler, and a lower end extended downward in the slag exhausting chamber, wherein the gas guiding pipe has a cooling water passage formed in a wall of the gas guiding pipe, a water inlet (N4a) and a water outlet (N4b) which are communicated with the cooling water passage respectively, characterized in that the positioning member comprises: an annular trough mounted on the inner bottom wall of the outer shell around the outer shell outlet and defining an annular groove; and an annular insertion plate defining an upper end mounted on an outer bottom wall of the inner shell around the inner shell outlet and a lower end inserted into the annular groove. an outer shell having an outer shell inlet formed at a top of the outer shell and an outer shell outlet formed at a bottom of the outer shell; an inner shell, which is disposed in and spaced apart from the outer shell, defines a gasification chamber therein, has an inner shell inlet corresponding to the outer shell inlet and formed at a top of the inner shell, and an inner shell outlet corresponding to the outer shell outlet formed at a bottom of the inner shell, and is fabricated by a membrane wall having a cooling water inlet and a cooling water outlet; a nozzle disposed at the tops of the outer shell and the inner shell so as to extend into the gasification chamber through the outer shell inlet and the inner shell inlet; a lower shell connected with a lower portion of the outer shell, defining a slag exhausting chamber therein, and having a slag exhausting port formed at a bottom of the lower shell and a gas discharging port formed in an upper portion of a side wall of the lower shell, wherein the gasification chamber is communicated with the slag exhausting chamber via the outer shell outlet and the inner shell outlet; a cooler connected with an outer bottom wall of the outer shell around the outer shell outlet, and having a cooling passage formed therein, a cooler water inlet, and a cooler water outlet; a positioning member disposed between the inner shell and an inner bottom wall of the outer shell; and a gas guiding pipe defining an upper end connected with the cooler, and a lower end extended downward in the slag exhausting chamber, wherein the gas guiding pipe has a cooling water passage formed in a wall of the gas guiding pipe, a water inlet (N4a) and a water outlet (N4b) which are communicated with the cooling water passage respectively, characterized in that the positioning member comprises: an annular trough mounted on the inner bottom wall of the outer shell around the outer shell outlet and defining an annular groove; and an annular insertion plate defining an upper end mounted on an outer bottom wall of the inner shell around the inner shell outlet and a lower end inserted into the annular groove. an annular trough mounted on the inner bottom wall of the outer shell around the outer shell outlet and defining an annular groove; and an annular insertion plate defining an upper end mounted on an outer bottom wall of the inner shell around the inner shell outlet and a lower end inserted into the annular groove.