The utility model relates to a combined type welding backing unit which comprises a supporting block (1), a filler block (2) and a positioning bar (3). The filler block (2) is placed in a groove formed in the top of the supporting block (1) and can be freely replaced. Due to the structure, the combined type welding backing unit can be combined randomly according to different welding environments, only the filler block (2) needs to be discarded after the welding backing unit is used, the supporting block (1) can still be continued to be used, the use efficiency of a welding backing is improved, and material wastage is lowered.