Abstract: Considering the effect of groove on distribution of welding heat input and geormetric feature of weld cross section, the heat source model for multi-pass welding of thick ultrafine grained Q460 high strengh steel plate was developed. The shapes and sizes of weld bead and HAZ were simulated by using ANSYS software,which agreed well with the experimental data. The thermal cycles in multi-pass welding of ultrafine grained steel were computed and quantitatively analyzed. The results show that subsequent weld passes have an important influence on peak temperature,retention time of high temperature and cooling time of thermal cycle in HAZ along the whole thickness direction of workpiece,and thus the heat input for subsequent weld passes or temperature difference among two adjacent weld passes should be controlled strictly.