Abstract: To improve the gain and stability of output microwave，an X-band high power coupled cavity traveling wave tube(CCTWT) with one RF sever and two RF lumped attenuators is studied. As an example of dot frequency microwave input，the large signal beam-wave interaction at 7.2 GHz of this X-band CCTWT is simulated using 3D PIC code. The simulated results show that this tube can produce an output power of 2.0 kW，an electron efficiency of 14.7% and a gain of 23 dB with a cavity number of 40，a beam voltage of 17 kV and a beam current of 0.8 A.