Abstract: The penetration control is the domain field on welding research using topside temperature information of weldment,and the choosing of the top temperature measurement points is also regarded as an important task of the real time control.In this paper on the base of the established three dimentional analytical model in a plate of finite size with a Gaussian distributed moving heat source the choosing fields of top face temperature measuring points for control of the back face width is quantificationally obtained making use of the relativity theory and considering the temperature sensitivity and the seam width.The general principle of the choosing fields is provided,and the next penetration control can be based on it.