Tree trunk liming is adopted in several nations to avoid the intrusion of pests and fungus. This study presents the design details of an automatic tree coating machine that can paint lime solvent on small and medium diameter tree trunks. The designed automatic tree coating machine included rack, elevation device, mobile nozzle bracket, mobile components, spraying device, and control system. The optimum position of the spraying nozzle was controlled by CPU based on the speed of the nozzle and the trunk diameter. The simulation showed that the design of the automatic tree coating machine can meet the requirements of deployment. The field experimental results indicated that tree trunks whose inclination angles are 85°–90° were painted within a single spray cycle, whereas the trunks with inclination angles of 75°–85° required multiple spray cycles. The spraying efficiency during the experiments was 130 trees/h, whereas the efficiency of traditional manual spraying was 5 trees/h per person. Therefore, this design is considerably efficient and advantageous.