The invention provides a gas welding nozzle which comprises a nozzle tube and an outer sleeve arranged on the nozzle tube in a sleeving mode. The gas welding nozzle is a silicon nitride ceramic tube. The nozzle tube comprises an inlet section, an intermediate section and an outlet section arranged in sequence, wherein the outer surface of the inlet section is cylindrical, and the part, near the top end, of the outer surface of the inlet section, extends outwards to form an annular flange; an inner hole of the inlet section is shaped like a frustum that tapers to the intermediate section; the intermediate section takes the shape of a cylinder whose circumference is smaller than that of the inlet section; the inner hole diameter of the intermediate section is equal to the diameter of a hole in the top end of the inlet section; the outlet section includes a connecting section connected with the intermediate section and a final section; the outside diameter of the connecting section increases gradually from the intermediate section; the outer surface of the final section is cylindrical; the outside diameter of the final section is equal to the maximum outer diameter of the connecting section; and the inner hole of the outlet section comprises a first section the same with the inner hole of the intermediate section, a second section gradually extending outwards and a third section which is an extension of the second section and has an inner diameter the same as the maximum inner diameter of the second section. The gas welding nozzle has the advantages of good wear resistance, high thermal conductivity and longer service life.