Abstract: Using manufactured fluorine alkali type sintered flux and the dedicated wire H03MnNi3MoTiB to weld X100 pipeline steels by double-wire automatic two-sided submerged-arc welding at different heat inputs, and then the detachability of the flux and the phases of slags after welding were analysed. The results show that when welding materials and welding methods were constant, the slag detachability was good at the weld heat input of 22.47 kJ·cm-1. After welding at different weld heat inputs, the main components of the slags consisted of CaF2, MgO, Al2O3, MnO and SiO2, in addition, the new phases MgAl2O4 and Mg2SiO4 were obtained. Although there were spinel-type compounds in the slag, its slag detachability was still excellent in the appropriate weld heat input of 22.47 kJ·cm-1.