The invention discloses a technology for catalysis and quality improvement by using waste gasified gases. A catalyst used for catalytic cracking of tar in gasified gases is selected from bottom slag in a domestic waste fluidized bed incinerator, crushed and activated bottom slag has a certain aperture structure, and calcium oxide and waste iron in the bottom slag have a catalytic cracking effect on the tar. The waste gasified gases are in full contact with the bottom slag catalyst in a multi-hearth furnace, the tar undergoes thermal cracking and catalytic cracking on the surface of the catalyst, and solid particles delay in the multi-hearth furnace. H2 generated after cracking of the tar reduces iron oxides in the catalyst into elemental iron, and CO2 and water vapor in the gasified gases react with carbon deposited on the surface of the catalyst to maintain the activity of the catalyst. The technology fully utilizes the bottom slag of the domestic waste fluidized bed incinerator, realizes waste recycling, realizes decomposition of dioxins in the bottom slag, also realizes recycling of the catalyst, purifies the waste gasified gases, and converts the tar into inflammable gases.