Abstract: According to the requirements of S-band crossed field amplifier (CFA) with long pulse width and large duty cycle, the dispersion and coupling impedance of the Stub Supported Meander Line structure are optimized. Then, the high frequency structure and energy output structure are designed and improved. Compared cold test result with simulation result,the performances of broad bandwidth and high efficiency are attained. 99.5% alumina ceramics is firstly used for insulation between control electrode and anode. This structure could avoid the breakdown phenomenon of insulation assembly during the condition of large average current. Based on Solid State Amplifier and two stage CFA, triple amplifier chain system is applied for hot test of large duty cycle S-band CFA. With the condition of a pulse width of 300μs and a duty cycle of 8%, the pulse power and average power are more than 250kW and 20kW respectively,with the bandwidth of 300 MHz , the operation efficiency is larger than 52% in full bandwidth. Basing on the breakthrough of key technology of high average power and long pulse width CFA in design and technological process, the technology of S-band high power CFA is improved drastically.