Abstract: A key technology issue of high-average-power free-electron laser (FEL) is the demonstration of stable, reliable, high-brightness, high-power electron source operation. The photocathode DC high voltage electron gun requires extreme-height-vacuum and more than 250 kV operating voltage. The vacuum vessel is a high conductance design, and the pump ability of NEGs can act well. To realize high voltage operating stability, the electrode system introduces a middle-potential electrode and the electric field strength can be depressed to 70 percent on the cathode support tube surface. Based on the design thoughts, the vacuum has been up to less than 510-9 Pa and the operating voltage reaches 300 kV without abnormity. In this report, we concentrate on the description of considerations for engineering design and some details of the photo-DC-Gun that have been proven fitting in practice.