The delay theoretical analysis result of single-longitudinal-mode pulse injected in active Fabry-Perot cavity has been obtained. The gain in Fabry-Perot cavity has been calculated and the numerical simulation result of output pulse has been obtained. The results show that when the energy of the pulse decreases at a certain value, the gain produced by increased pump particle population can compensate the loss in the cavity and the delay pulse can achieve steady state. In experiment, the active Fabry-Perot cavity is a Nd: YAG laser sided pumped by laser diode. For the pulse inputed externally, we achieve 140 pulses within 2 ups. Finally, the reference light is delayed using the active Fabry-Perot cavity. And we show the heterodyne experiment of signal light whose frequency shift generated by an acousto-optic modulation (AOM), and the experiment results accord with the frequency shift of the AOM and the error is less than 4%.