According to the theory that thermal expansion in the weld is affected by the defect in it, finite element analysis software ANAYS is used to compute the temperature distribution on surface and analyze the unnormal temperature distribution caused by foreandaft crack defect. Different unnormal temperature distribution is caused by the crack of different length, width and depth ,which is in different position of weld. The emphasis is placed on the quantitative relationship between the dimension and position of defect, the surface thermal character and the consistence of heat flux imposed. This job can provide method for infrared nondestructive testing be used for the testing of crack in the weld and foundation for the establishment of exact and reliable testing standard.