Abstract: In order to research the generation technology of electron beams with long pulse and strong current, a high voltage long pulse generator has been designed. The generator contains six parts: Marx generator, charging inductance, pulse-forming line, high voltage switch, pulse transmission line and plane diode. The inner conductor of the pulse-forming line is spiral line, and the working material is deionized water. The indexes of this generator are output voltage 500 kV, current 40 kA, pulse width 1 μs and single pulse energy 20 kJ. The paper mainly introduces the structure and electric parameters of the long pulse generator, and discussies the issues which should be noticed in the designs. At present, the generator is under process.