The invention discloses an inward-pushing-type clamping device of a three-dimensional corner clamping tube in a numerically-controlled tube bending machine, and belongs to the field of numerically-controlled tube bending machines. The inward-pushing-type clamping device comprises a flexible chuck, and further comprises a top end fixing sleeve, an outer fixing sleeve, a hydraulic outer cylinder, a bearing, an inward-pushing sleeve, a hydraulic inner cylinder, a first hydraulic cavity and a second hydraulic cavity. The top end fixing sleeve, the outer fixing sleeve, the hydraulic outer cylinder, the bearing, the flexible chuck, the inward-pushing sleeve, the hydraulic inner cylinder, the first hydraulic cavity and the second hydraulic cavity are respectively of a rotationally-symmetric structure, share the same central axis and internally forms a clamping inner cavity jointly. Due to the arrangement and assembling relationship of the components, when the inward-pushing-type clamping device clamps tubes, clamping force is easily controlled, tubes are not damaged, it is ensured that the inward pushing sleeve is steadily pushed to move and the flexible chuck is steadily pushed to contract in the clamping process, and the damage and skid, caused by uneven moving, of a workpiece are avoided.