Relating to a tobacco machinery, the invention discloses a tobacco dust flat bag horizontal insertion, off-line deashing and dedusting unit modular combined deduster unit. A plurality of unit vertical partition boards are vertically fixed in a unit box inner cavity and separate the unit box into a plurality of dedusting unit module box bodies that are mutually isolated. Each dedusting unit module is provided with a flat type filter, a gas circuit switch valve group, a pulse-jet system, an ash bucket and an ash discharge valve that are respectively independent. Each dedusting unit module box body is equipped with a to-be-purified air inlet, a purified air outlet, an observation window and a repair door that are respectively independent. When the deduster operates, a working mode that each dedusting unit module undergoes off-line deashing in order is employed, and a flat type filter no longer filters dust-containing gas during deashing, so that the deashing system can be at an ideal state, and the deashing effect is good, the efficiency is high, and the energy consumption is low. The dedusting unit module that does not undergo deashing and dedusting operates continuously, so that the combined deduster unit can be always at an efficient and stable operating state.