Abstract: A compact high-power(GW level) nanosecond pulse generator is developed. The pulse generator includes a 40 coaxial pulse forming line charged by a built-in Tesla transformer with open circuit magnetic core, primary circuit and high-voltage gas switch with gas circulating between the electrodes. The generator has characteristics of high transformer ratio, compact structure, high energy transfer efficiency, and convenient for high repetition rate operation. The operation principles, design methods and simulated calculations of pulse forming line, Tesla transformer and main switch are described. The experimental results show that the output voltage of the pulse generator is more than 200 kV with a width of about 8 ns, and it can operate steadily under the condition of repetition rate of 100 Hz and average output power of 1 GW, the experimental results are in good agreement with that from theoretical design.