Abstract: In this paper, the design process and experimental results of a beam optical system (BOS) for a 220 GHz folded waveguide (FW) backward wave oscillator (BWO) are demonstrated. The BOS is comprised of three parts: the Pierce convergent electron gun, the magnetic focusing transmission section and the single-stage depressed collector. For a vacuum electronics device operating in terahertz frequency (THz) regime, the ultra-small and long beam tunnel makes it tough for electron beam to pass through at a high rate. The space charge effect, the machining and assembly precision, the thermal velocity spread and thermal expansion will all make things worse. Therefore, to develop a practical BOS with sufficient passing rate is an urgent but difficult task for the development of the 220 GHz FW BWO. Using theoretical method and numerical tool, the three parts were designed one by one, to satisfy the requirement proposed by earlier analysis on beam-wave interaction. Based on the design results and error analysis, the sample tube of BOS was preliminarily developed and tested. The experiment data show that the BOS can generate an electron beam with a beam voltage of 15.4 kV, a beam current of 22 mA to pass a beam tunnel with a diameter of 0.19 mm, a length of 30 mm, where the passing rate is over 80%.