Abstract: A discharge fault occurred on the basin insulator of gas insulated metal enclosed switchgear in a 220 kV substation. The causes of the discharge fault were analyzed by means of macroscopic inspection, penetration detection and X-ray detection. The influence of different pouring gaps on the electric field under high voltage environment was calculated by simulation software. The results show that the pouring gap would distort the electric field of the basin insulator. Under the condition of equal gap depth and arc length, the smaller the width of the pouring gap was, the greater the distortion of the electric field was, and the easier the gap was to be broken down, so that the basin insulator was prone to flashover.