Partail discharge (PD) of the gas insulated switchgear (GIS) is the outside phenomenon of its insulation defect. In order to study the PD development, an artificial insulation defect model of surface free metal particles on basin-type insulator in an experimental cavity of a 220 kV GIS is set up. The PD development stages under the defect are simulated with step-up voltage method, and the PD signals in each stage are measured based on UHF method. Based on the measured data, the variations of the discharge frequency, the average amplitude of discharge, the total discharge amplitude, and the maximum discharge magnitude are summarized, and it can be concluded that the PD development triggered by the insulation defect of surface free metal particles on the basin-type insulator of GIS can be divided into the initial stage, the developing stage and the dangerous stage. Besides, these stages can be recognized by the discharge characteristic quantity and discharge scattered point maps.