An automatic supplement system of milk for piglets in the farrowing unit of large-scale pig farms with homeothermia circulation was designed to realize the functions including preparing milk， feeding circularly， and cleaning automatically to solve the problem of low level of automation in nursing piglets in large-scale pig farms. The overall structure and working principle of the supplement system of milk for piglets were explained. The structural design and theoretical analyses of key components including the mixing tanks， stirrers， circulation pipelines， delivery pumps， and auxiliary heaters were conducted. A control system for the supplement of milk was developed with Siemens S7-200 SMART as the core processor. Human-computer interaction through a touchscreen was realized by controlling the flow state of the milk solenoid valves and relays， and monitoring the physical parameters of milk with flow sensors， temperature sensors， liquid level sensors， etc. A prototype was manufactured and the performance of prototype was tested. The results showed that the overall average coefficient of variation for the uniformity of the milk prepared by stirring was 0.435%， and the pipeline pressure ranged from 26.0 kPa to 65.2 kPa. All milk refilling bowls were filled with milk within 6.1 seconds， with the temperature of milk controlled between 40 ℃ and 42 ℃. It is indicated that the designed automatic supplement device of milk for piglets in the farrowing unit has good uniformity of milk， reasonable distribution of pressure， with the temperature of the milk meeting the requirements of supplementing milk for piglets， thereby satisfying the production needs of automatic supplement of milk for piglets in the farrowing unit.