The invention discloses a planar annular microstrip slow-wave structure which belongs to the field of microwave electric vacuum technology. The planar annular microstrip slow-wave structure relates to a traveling-wave tube amplifier device. The planar annular microstrip slow-wave structure comprises a dielectric substrate (2) and metal wires (1) on the surface of the dielectric substrate (2). The planar annular microstrip slow-wave structure is characterized in that each metal wire (1) has a periodical structure which is obtained through successively connecting a plurality of annular units with same shape and same dimension. Each annular unit is obtained through joining two open rings which are in mirror symmetry in a vertical direction. Two adjacent annular units are connected at a joining part between the two open rings through a microstrip line. Compared with an existing microstrip slow-wave structure, the planar annular microstrip is advantageous in that a vacuum device with the planar annular microstrip slow-wave structure can be used in a higher operating frequency band on condition of same dimension because of wider cold bandwidth. The planar annular microstrip slow-wave structure provided by the invention has a wide transverse dimension, thereby effectively reducing a requirement for an electronic gun and a focusing magnetic field. The planar annular microstrip slow-wave structure has a relatively high coupling impedance and can perform interaction with an electron beam in a relatively good manner. Therefore the planar annular microstrip slow-wave structure has relatively high potential and is suitable for miniature planar traveling-wave tubes.