Abstract: Based on macro heat transfer mechanism, a three dimensional numerical analysis model for temperature field in single wire FAB (flux aided backing) submerged arc welding of thick plate was established by considering the influence of groove on welding heat input and the geometry of the weld cross section. The temperature field and the thermal cycle in FAB welding of D32 steel for different thicknesses were calculated using ANSYS software and the thermal field was analyzed. The results show that the calculated weld cross section shape and size as well as the thermal cycle were in fair agreement with experimental data, indicating the accuracy and adaptively of the model. In the condition of good weld formation, with the raise of plate thickness, the width of HAZ at the top and bottom surface of the work piece decreased to some extent, but the cooling time t_8/5 of thermal cycle had no obvious changing tendancy.