By means of computer, processing on photos of laser welding plasma taken by high-speed photography at 1000 frame frequency is discussed . The area of welding plasma is calculated by the use of pixel and expressed as quantity of pixels , then the diagram of picture number to plasma area is given out . The ithothermal line based on gray scale is drawn . The propagation affection of laser induced plasma to incident laser is analyzed when the induced plasma is divided into 10 regions with different refractive index, the results show the diverging of laser beam make the induced plasma like negative lens and even welding unable to proceed in sever condition.