Abstract: The system composition and key technology of a millisecond long pulse klystron transmitter are introduced in this paper. According to the development of domestic and foreign technology, as well as the requirement of missile range instrumentation radar, system design of the transmitter is determined. The advantages and disadvantages of the modulation assembly output superposition modulator and insulated gate bipolar translator series modulator are analyzed. The principles of BOUNCER compensation circuit and multi-power supply temporal compensation circuit are presented particularly. The simulation and test results are given, and the two pulse top drop compensation schemes are analyzed comparatively. The millisecond long pulse transmitter based on the optimal scheme is measured lastly. It is demonstrated that all the specifications such as output power, envelope and frequency spectrum can meet the requirements of design specifications.