We have investigated the electron beam transit-time effect in the π-mode standing wave electric field in the nonuniform two-cavity and three-cavity oscillator under the small signal condition. Mathematical expression of the beam-to-microwave power conversion efficiency has been obtained. 研究了小信号条件下,电子在非均匀两腔和非均匀三腔谐振腔π模驻波场中的渡越时间效应,得到了束波功率转换效率的函数表达式。