Abstract: A general waveguide pill-box window could provide maxim relative bandwidth no more than 30% because of the ghost mode effects. A new full band waveguide high power asymmetric pill-box window without ghost mode simulation and optimized approach is presented. This type pill-box window has broad band, no ghost mode and excellent voltage standing wave ratio(VSWR) characters and specially suits for high power helix TWT with kW level output. An L-band pill-box window 3-D simulation prototype connected with impedance match waveguide is obtained according to this method, which has a relative bandwidth up to 41.4% without ghost mode and the VSWR is lower than 1.1．