Abstract: Traditional transit-time oscillators such as split-cavity oscillator, three-cavity oscillator (TCO), super-reltron etc., have a common characteristic that the span between foils in the modulating cavity is equal. By varying span from equal to different, we developed a new device called nonuniform three cavity oscillator (NTCO). Based on the motion of the single electron whose incident phase is φ0 in standing wave electric field under the small signal condition, the transit-time effect of electron beam in π mode standing wave electric field in the NTCO is investigated, function expression of beam-wave power conversion efficiency is obtained, constrained optimization problems are solved through “penalty function method”, the conclusion is obtained that beam-wave power conversion efficien