An S-band 50 MW klystron (Fig. 4) was successfully developed by the Institute of Electronics, Chinese Academy of Sciences (IECAS) for linear accelerators. A structure of six-cavity and single output window was adopted, and three prototypes were fabricated and tested before November of 2005. Although the first tube suffered from short circuit of heater as reaching operating voltage of 220 kV, the second and the third ones have made a success. For the third klystron, under the conditions of an RF pulse width of 4 mus, a repetition rate of 50 Hz, a beam voltage of 305.9 kV and a beam current of 368 A, the peak output power is 51.4 MW, efficiency is 45.6%, and gain is 54 dB. This paper describes the design ideas, simulation and test results.