The invention discloses a high-linear-speed rotating magnetic liquid sealing device with a small radial size, and belongs to the field of mechanical engineering sealing. The device is especially suitable for sealing of high-linear-speed rotating working conditions. The problems that when the linear speed is low in the rotating shaft starting process, an existing magnetic liquid centrifugal sealing structure is insufficient in pressure resisting capacity and low in reliability are solved. The device is composed of a rotary shaft (1), a shell (2), a left bearing (3), a first magnetic isolating ring (4-1), a first pole shoe (5-1), a first sealing ring (6-1), a first permanent magnet (7-1) and the like. When the rotary shaft rotates at low linear speed, the device achieves a sealing function through an axial magnetic liquid sealing assembly, under the condition that axial space is sufficient, the number of sealing teeth of axial magnetic liquid can be increased, the pressure resisting capacity can be improved, and therefore the reliability of sealing is improved, and the problem that the existing magnetic liquid centrifugal sealing structure is insufficient in pressure sealing capacity in the rotary shaft starting process is solved. Meanwhile, the pressure resisting capacity of centrifugal sealing in the high-linear-speed rotating process of the rotary shaft (1) is improved.