The invention relates to a lawn trimming device, and belongs to the field of agricultural mechanization engineering. In order to overcome deficiencies of current lawn trimming devices, the invention discloses a planetary lawn trimming device. The technical scheme for realizing the above invention is that a motor is utilized to drive a sun gear in a planetary gear mechanism to rotate, and planetarygears are subjected to rotation and revolution through control of gear rings and a planetary frame, so that a blade below planetary gear shafts is driven to perform mowing work. The planetary lawn trimming device disclosed by the invention has the following advantages: (1) the device adopts planetary gear train transmission, has a compact structure, works stably and has high working efficiency; (2) the blade is installed below the planetary gear shafts, when the planetary gears are revolved, the blade can be driven to cut a curved zone or a circular ring zone on a lawn, so that the purpose ofbeautifying the lawn is achieved; and (3) the position of the blade can be adjusted, so that realization of different-degree trimming of the lawn is facilitated.