The microwave source for spatial power combining based on single beam klystron has been developed, which will meet the requirements of modern radars and high power equipments. The system composition, high power klystron, phase control and high power test system of the 50 MW super-high power klystron transmitter developed are introduced. The paper puts emphases on the key technologies of the transmitter， and provides some concrete solutions to the design difficulties. The design of the inductive-adder high-power all-solid-state modulator based on high-power IGBT modules as switches is presented, which can improve the reliability of the transmitter. The test results of the super-high power transmitter are given, which show that the transmitter satisfies technical requirements, and improvement methods for the transmitter are discussed. The future development of the microwave source for spatial power combining based on single beam klystron is forecasted.