Abstract: Through the 2.5D particle-in-cell simulation code Karat, an X-band Cerenkov-type oscillator without guiding magnetic field is investigated by us. The operating frequency of the oscillator is determined by the cold cavity dispersion curves and basic structure is built by the microwave distribution simulation software, Superfish . Based on the basic structure of the oscillator, the optimizations are worked by adopting the inner and outer constraining cathode circles to improve the transportation quality of the electron beam, tapering the collector to optimize the Q-factor and purify the microwave mode, and optimize the parameters of the nonuniform slow-wave-structure to enhance the beam-wave interaction. The efficiency of the oscillator is optimized to 30% from 15%, and the output wave power is 1.8GW with a frequency of 9.1GHz in the TEM mode. The corresponding diode voltage and beam current are 620kV and 9.4kA, respectively.