The work aims to reduce the milling difficulty of micro parts with TC4 titanium alloy, and improve the surface machining quality and efficiency of TC4 parts. The electrolyte with NaAlO2 as the main component was designed, and a porous plasma electrolytic oxidation (PEO) coating with thickness of about 20 μm was grown on the surface of TC4 titanium alloy in situ by means of electrolytic plasma oxidation. The structure and composition of the PEO coating were characterized by scanning electron microscope (SEM) and energy dispersive spectrum (EDS). Meanwhile, the cutting force and surface roughness of the PEO coating during micro milling were measured by force measuring instrument and white light interferometer respectively. The results showed that the PEO coating is obtained by in-situ growth of TC4 titanium alloy, and its thickness is about 20 μm. Furthermore, the structure of PEO coating is loose and porous, the porosity is higher, the distribution of pores is uniform, and the binding force with the matrix is poor. Compared with TC4 titanium alloy, the elastic modulus and hardness of PEO coating were reduced by 79.8% and 75.0%, respectively. Under the same cutting parameters, the three-dimensional milling forces were decreased by 91.90%, 78.13% and 42.62%, respectively, the surface roughness Ra was reduced by 52.6%. Consequently, compared with the traditional film, the PEO coating fabricated by electrolytic plasma has a more porous structure and lower strength, which can significantly reduce the three-dimensional force of micro milling, and improve the surface roughness without top burr. This method significantly reduces the machining difficulty of micro milling for TC4 titanium alloy, improves the machining surface quality effectively, and verifies the feasibility of the plasma electrolytic oxidation method used in the auxiliary milling of TC4 titanium alloy.