In order to meet the multiple requirements of high safety， low cost， high temperature resistance， and high pressure resistance on the seismic source for deep oil exploration， a kind of high-voltage switch consisting of two gas discharge tubes （GDTs） was used to control the discharge of thin film capacitor， and S-type bridge foil was exploded into high-temperature and high-pressure gas/plasma to ignite the boron potassium nitrate （BPN） pellet and generate underwater shock waves. Then， the ignition threshold of the seismic source was determined by the up-down method， and the acoustic characteristics were also studied using pressure probe. The results show that the firing unit composed of a thin film capacitor （2 μF） and the high-voltage switch based on GDTs can reliably achieve pulse discharge， and work at the extreme underground environments such as high temperature and high pressure， and the cost is less than ¥10. Using the firing unit to stimulate the S-type Cu bridge foil， a firing voltage threshold of 1100 V and a critical peak current of 1847 A were determined. The measurements of underwater shock waves and acoustic source level （ASL） analysis show that the ASL excited by the seismic source is higher than 150 dB in the 50-800 Hz frequency band， meeting the requirements of underground seismic sources.