Control rod drive mechanism Canopy-seal weld mockup of PWR was taken as the object of study. The microstructure of oxidation film and the oxidation mechanism on the surface of 304L base metal near the weld, under the gas tungsten arc welding without inert gas shielding on the base metal, were investigated. The morphology, composition and structure of the oxide film were characterized by optical microscopy, scanning electron microscopy, energy disperse spectroscopy, laser Raman spectroscopy, Auger electron spectroscopy, transmission electron microscopy and X-ray photoelectron spectroscopy. With decrease of distance to fusion line, oxygen content on the surface of the base metal increased, the size of oxide particles grew up, and cracks appeared on the oxide film, gradually. In the region at 2 mm distance from the fusion line, surface oxide film was mainly Cr2O3 and Cr-O amorphous. In the region at 7 mm distance from the fusion line, surface oxide film was mainly comprised of Fe3O4 with a few FeCr2O4 and Cr2O3. The temperature on the base metal during welding affected the growth rate, particle size, oxide species of oxide film on the surface of base metal. When the oxygen potential of the metal atom and the partial pressure of ambient oxygen are determined, the influence of temperature on diffusion is the main factor determining the composition of the oxide film.