A coaxial transit-time oscillator with travelling-wave output structure is proposed. The extractor of this device adopts an extended interaction cavity loaded with washer-like structure, which has the advantage of high beam-wave interaction efficiency and low space charge potential energy. The extractor operating at 3π/2 mode not π-like mode improves the group velocity of wave, which is favorable for RF extraction. The loaded quality factor of the buncher is increased by introducing a prior reflector, which can obviously reduce the starting time of microwave. The device is designed and optimized by a PIC (particle in cell) code. With the voltage 530 kV，the current 12.8 kA and the guiding magnetic field 0.7 T, the output power is 2.41GW at the main frequency 7.76GHz, and the efficiency of beam-to-microwave power conversion is about 35.5%.