Abstract: Based on the characters of weld thermal cycle, the hydrogen traps thermal desorption model is founded and studied by using the numerical analysis. The results indicated that the hydrogen traps have the process of thermal desorption when the previous weld bead is heat treated by the latter bead in the HSLA steel multilayer and multipass welding process, and all of hydrogen in weak traps has almost been desorbed in the heating time section, but the hydrogen in strong traps has only little been desorbed in all the time of weld thermal cycle. The hydrogen traps thermal desorption proportion will increase when the parameters of interpass temperature, peak temperature and cooling time increase in weld thermal cycle, but it will decrease when the parameter of heating rate increase. The process of hydrogen traps thermal desorption is useful to dehydrogenate in HSLA steel multilayer and multipass welding, and it can effectively decrease the probability of hydrogen induced crack.