Abstract: This paper introduces  the development of a new ultra-wideband (relative bandwidth ≥17%) S-band multi-beam klystron (MBK) for the first time in China. By optimizing the electronic optics system, gun construction, high-frequency system, heat removal system and so on, the electronic passing rate can be improved and the heat dissipation pressure in the high frequency part can be reduced. These measures enabled the MBK to achieve 120 kW peak power with 30% efficiency and 40 kW average power. At the same time, through low cathode load design and many process measures, the steady working with 3 ms pulse length and continuous 24 h of steady work have been realized. To achieve full power output with in 3 min and 2000 h life time, we have optimized the working parameters of cathode and structure and technology of electron gun for many times. This project has completed the all index, the small-batch production and vehicle-mounted environmental testing, with engineering application conditions, which can provide effective technical reference for subsequent similar designs.