The invention discloses a three-dimensional roof controlling method for fully mechanized caving of a steeply inclined and extremely thick coal seam. The steeply inclined and extremely thick coal seam is mined in multiple sections from back to front along the trend of the coal same, each of the mining sections is cut into multiple sub-layers from top to bottom, and the uppermost sub-layer is a top sub-layer; when any mining section is mined, the multiple sub-layers of the mining section are respectively minted from top to bottom, and the process comprises the following steps: I, mining the top sub-layer: comprising the steps of tunnel construction, extraction of toxic and harmful gas in a pre-denotation roadway, advanced pre-blasting of top coal, drainage to wet the top coal, coal mining and nitrogen injection behind a working face and ground backfill of a subsidence area; II, mining the next sub-layer; and III, repeating the step II once or multiple times till the mining processes of all sub-layers of the mining section are completed. The method disclosed by the invention is simple in step, reasonable in design, convenient to implement and good in using effect, and various protective measures are combined in use, so that the potential safety hazards such as roof caving, gob fire and the like can be effectively eliminated.