This article focuses on the development of a trigger source for a three-electrode flat triggering vacuum switch with a small electrode gap of less than 10mm. The selection and verification of circuit components for the trigger source were conducted, and it was found that the trigger source circuit can output a peak pulse voltage of up to 12.25kV and a rise time of 7.9μs under no-load conditions. Subsequently, the trigger source was connected to the trigger electrode, resulting in a breakdown voltage of approximately 9.5kV at the trigger electrode. Through multiple repeated experiments, it was determined that the trigger source can reliably trigger the trigger electrode at a trigger voltage peak exceeding 9.5kV.