The in-vitro degradation behavior and corrosion mechanism of Mg-4.0Zn-0.2Mn-0.2Ca (wt%) micro-tube was investigated by the immersion tests and electrochemical tests. The results show that the corrosion resistance of the micro-tubes can be improved by the annealing treatment. The long-term immersion tests reveal that the corrosion process is relatively uniform, and the corrosion rate of the annealed micro-tube in Hank's solution is about 0.30 mm/a. During the initial stage of immersion, Mg(OH)₂ is formed on the surface of the annealed micro-tubes, forming a protective film to hinder the corrosion progress. Although the formed hydroxyapatite (HA) on Mg(OH)₂ film can further reduce the corrosion rate, the coarse secondary phases in Mg matrix can enhance the galvanic corrosion effect. The generated abundant hydrogen may destroy the HA film, thus promoting the corrosion process.