In this paper the image by high-speed photography of plasma in CO2 laser welding is studied including the area of these images, the change rate of these images, the isogray line of the image and the maximal variation of the image gray. The used laser is RS850 made in German and the high-speed photography is NAC-10 made in Japan. The weld material is low carbon steel. The welding parameters include laser power 4KW, welding speed 1.2m/min, shielding gas Helium, Helium flow rate 11L/min. The parameters for high-speed photography are as exposure time 1/5000 of second, shoot frequency 1000 frame/s. According to the analyses the main conclusion as follows: In the experiment, the values of gray of these images cover from 40 to 255. The area of the plasma is oscillation and the average frequency of the oscillation is about 300Hz. The laser welding plasma can be divided to three parts: periphery, smoothness and core from the external to inner. The isogray line of the periphery is very irregular because of shocking of the shielding gas and the metal spatter. In the core region, the thermal motion of the electrons is violent, so there is lots of little division with complex shape. The gap of isogrey line in the periphery region and core region are larger than it in the smoothing region. The isogrey lines of the image in the melting pool link with the isogray line of the image of the laser welding plasma, so it can be used to checking the temperature field each other. There exits an isothermal kernel in the core region.