The invention discloses a transonic speed movable blade top gap active control device. When the working condition of a transonic speed movable blade turbine is changed, due to the flow field transonic blocking effect, the movable blade close-front-edge temperature and a front edge blade top gap are not influenced by changes of the expansion ratio, the movable blade close-tail-edge temperature and a tail edge blade top gap are changed along with changes of the expansion ratio, and the expansion amount needed by the position, corresponding the movable blade tail edge, of a case is larger than the expansion amount needed by the front edge position. Cooling ribs at different heights are distributed at the positions, corresponding the movable blade front and tail edge, of the case, and are each surrounded by two cold air cavities, and the height of the cooling rib corresponding to the movable blade tail edge position and the number of cooling holes in the cold air square cavities are both larger than the height of the cooling rib corresponding to the movable blade front edge position and the number of cooling holes correspondingly; and the cold air amount in cold air branch pipes at the upstreams of the two cold air square cavities can be independently controlled, control over the blade top gap heights of different axial positions of a movable blade is finally achieved, and the working performance and safety of the transonic speed movable blade turbine are effectively guaranteed.