Three-electrode field-distortion gas switch is a crucial element of modular fast linear transformer driver (FLTD). Electrode erosion affects the trigger jitter during the lifetime of the switch, which in turn can affect the output characteristics of FLTD. Therefore, studying the impact of electrode erosion on the trigger jitter of the switch is of great significance to optimize the switch structure and predict the switch life. This paper studies the erosion characteristic of intermediate electrode of three-electrode switch, and the electrode materials are stainless steel and brass. The key factors affecting the lifetime of switch are obtained by considering the changing rules of trigger and erosion characteristics, which provides theoretical support for the optimization of the performance of the three-electrode switch. The results show that the erosion area and surface roughness of stainless steel and brass electrodes increase with discharge times. The brass electrode is ablated more seriously and the stainless steel electrode has higher surface roughness. With the increase of discharge times, the breakdown point moves to the electrode edge area, which affects the insulation performance of the switch.