Abstract: Displaying welding gas flow by laser is a new method developed in recent years.Study on the principle of image formation is necessary because of the complexity of the image fringes.In this paper the effect of the refractive index on image formation has been studied with experimental method,and shown that this display method issensitive in the abrupt region of the refractive index.Interface of the gas flow is clearly displayed,thus a gas flow pattern is formed,and the flow field behavior can be determined.The characteristics of the distribution of gas flow diffractive fringes have also been studied.Bright fringe appears on the side of the larger refractive index of the gas flow interface,while dark one appears on the side of smaller refractive index.Hence it is possible to determine the gas on either side.Guided by these principles,analysis and determination have been made of the image characteristics of free jetflow,impact flow and electric arc impact flow.On the basis of the study of the effect of the phototaking distance on image formation,the optimum phototaking distance of 1.5-2 m is recommended.