Cryogenic machining of titanium alloys using the internal cooling method is being identified as an alternative effective process to current practice of machining materials with poor thermal conductivity. The choice of jet parameters is particularly important for improving their machining quality and saving the production cost simultaneously. This research aimed to minimize the surface roughness by optimizing the comprehensive jet parameters in cryogenic milling Ti-6Al-4V. By comparing the cooling capability of liquid nitrogen and gaseous nitrogen, the influence mechanism of nitrogen phase on surface roughness was illuminated. A self-developed cryogenic machine tool with conveying liquid nitrogen through the spindle and tool was specially used to carry out milling experiments. The results indicated that the nitrogen phase had a most significant effect on surface roughness, followed by the pressure while the effect of flowrate was lowest. A lower volume fraction of gas, a higher pressure, and a proper flowrate could produce a lower surface roughness. An optimal combination of jet parameters was ultimately selected as the liquid nitrogen with 45 l/h of flowrate and 0.6 MPa of pressure to obtain the minimum surface roughness at 0.076 μm.