The invention relates to a method for improving quality of a vacuum fifth line product of a lubricating oil type vacuum separator tower, and a novel separator tower is introduced in a conventional lubricating oil type vacuum tower. A lower right angle separator is adopted, one side of the vertical section of the separator is used for feeding, the other side of the vertical section of the separatoris used for discharging, the horizontal section of the discharge side of the separator is connected with a tower wall by welding, and the separator tower is divided into three working sections, respectively comprising a pre-fractionation section, a common rectification section and a vacuum fifth line collection section. Normal bottom residue oil to be separated is heated by a heating furnace, enters from the middle of the pre-fractionation section, and is stripped by tower bottom steam, after the stripping, a liquid phase as vacuum residue oil is discharged from the bottom of the tower, a gasphase is sent to the common rectification section, and further separated by a side stripping tower to obtain product diesel and vacuum second line, vacuum third line and vacuum fourth lubricating oil. A stream of steam is introduced in the lower part of a vacuum fifth-line collecting section for stripping, and the vacuum fifth-line lubricating oil product is collected at the bottom. Under the premise of no increasing of the energy consumption of a vacuum furnace, the distillation range of the vacuum fifth-line lubricating oil product and overlapping degree with adjacent products are significantly reduced, and the method is used in the design of a new tower, also in the technical transformation of an old tower.