Spark-05 was built for conducting researches on medium/high impedance high power microwave sources. The interface between oil and vacuum was one of the keys to the system design. For reasons of compactness and simplicity, a disk type vacuum insulator was adapted. Electrostatic field along the insulator surface was calculated. By adjusting the position of the shielding rings, electric fields were well distributed, and the field around the triple junction was controlled. The insulator worked well in 1MV experiments afterwards.