A 24kV long pulse modulator is designed for DESY's TESLA Test Facility (TTF). This modulator, with a 120 kV, 1.7 ms width pulse output, is used to drive a Thomson TH1801 multibeam klystron. In order to make waveform flatness of the klystron voltage less than ±0.5%, it is necessary to use a bouncer circuit. This paper gives the Pspice simulation results of the 24 kV long pulse TESLA modulator.