As the key equipment in the power system, vacuum interrupter has been widely used in middle and low voltage occasions. During the manufacture and operation of vacuum interrupters, defects may be introduced, leading to a decrease in insulation performance. Partial discharge (PD) detection can timely find potential internal hazards and avoid equipment failures. However, there are few research on PD detection of vacuum interrupters. In this paper, a platform for 35 kV vacuum interrupter PD test is established, and the PD characteristics of a 2-mm long protrusion on the static contact are studied. The results indicate that significant PD occurs at 20 kV. The discharges are asymmetrically distributed in the positive and negative half cycle. The discharges in negative half cycle are lower than 5 pC, while discharges in positive half cycle can exceed 50 pC. As the test voltage increases, the discharge repetition rate and average apparent charge both increase. This study contributes to understanding the PD characteristics of defect in vacuum interrupter, laying a foundation for defect detection and diagnosis in vacuum interrupters.