In this paper.a kind of waveguide Gunn effect oscillator at 1.25cm is discussed. Its main cavity uses the structure of adjustable coaxial height reduced waveguide. This makes the circuit match the diode well. The paper puts stress on discussing the influence of the size of each part on the characteristic of the oscillator. High Q H₀₁₁ mode cylindrical cavity is used as the steady-frequence cavity and metal materials with different line-expand coefficients for temperature compensation is also used. These improve the frequience stead degree. In 23GH₂-25GH₂ band, using WT55 Gunn effect diode, the output power of the oscillator is larger than 30mw, the frequence stead degree is 5×10^-5.