Abstract: This paper presents the progress on an Lband long pulse relativistic klystron amplifier. Modulation of IREB through the idler cavity gap and microwave extraction from the modulated IREB are described in detail. The problem of beamcurrentpulseshortening through the idler cavity gap is solved to a certain extent as well as the problem of output microwave pulseshortening, and highmodulated IREB are obtained after parameter adjustment on the input microwave and the idler cavity. In the experiment, the 446kV/ 30kA/1.4μs hollow electron beams generated about 2.0kA/1.0μs modulated current after two idler cavity gaps. They got more than 381MW/200ns RF power with the new output cavity loaded Q of about 7.9. The efficiency is 28%, and the gain is 34dB.