A vacuum system of 3 GeV Taiwan photon source (TPS, with circumference 518.4 m), is designed for low-outgassing aluminium beam ducts, low-impedance structure, oil-less pumping system and oil-less fabrication. A prototype (total length 14 m) of the TPS vacuum system has been fabricated. This one-cell prototype vacuum system consists of two bending-magnet chambers (~4 m long each), two straight chambers (~3 m long each), supporting stands, five beam positioning monitors and pumps. Two bending-magnet chambers were made by computer-numerical control machining, which was lubricated with ethanol to protect the surface from oil contamination. Next, these two bending chambers were cleaned with ozonated water to decrease the rate of thermal outgassing and photo-stimulated desorption. The rest chambers were also cleaned by chemical cleaning method. An automatic welding system then was used to implement the side-welding seams of bending chambers in a temperature- and humidity-controlled cleanroom. The bending and straight chambers were welded into one piece by ‘on-site welding’. After baking at 150°C for 24 h, an optimizing pressure could be achieved at 4.9 nPa.