The invention discloses a radial fixed bed gasifier, an application thereof, and a gasification method of a carbon-containing substance. The gasifier includes a gasifier body and at least one ventilation tube. The gasifier body includes an inner-layer sleeve, and an outer-layer sleeve which sleeves the inner-layer sleeve in a sealed manner to form an interlayer chamber. A side wall, in the interlayer chamber, of the inner-layer sleeve is provided with at least one through holes. The side wall of the outer-layer sleeve is provided with a ventilation end port. The ventilation tube includes a ventilation portion and a gas transmission portion, each of which has a head end and a tail end. The tail end of the ventilation portion is connected to the head end of the gas transmission portion. The tail end of the gas transmission portion passes through the gasifier body. The ventilation portion is disposed axially in the inner-layer sleeve. At least one through hole is formed in the side wall of the gas transmission portion. The radial fixed bed gasifier can employ the carbon-containing substance being small in particle size, is equal in bed pressure drop with other radial fixed bed gasifiers, is clean and environment-protective, is high in productivity and is high in gasification efficiency.