Abstract: It is of wide value to calculate the restraint stress of weld bead in the slit-type specimens, a kind of specimens whose welding stress field are very near to the real structures. In this paper, a mathematical model of the contraction curves of the groove gap along the Slit has been derived from the calculated results by the means of the thermo-elasto-plastic FEM, and on this basis, combined with the restrant intensity distrubution function of the specimens, the relationship between the restraint intensity and the restraint stress in the slit-type specimens has been formulated for the first time. The calculated and experimented results show that the formulas can reflect the real stress level of the welded joints in actual structures and are valuable for welding practice.