The invention provides a high-stability output device for a periodically-modulated flat-topped pulse. The device comprises a polarized light source, an electro-optical switch, a reflective structure and a first polarization device, wherein the polarized light source is suitable for providing polarized light to be modulated; the electro-optical switch comprises an electro-optical crystal and an electro-optical driving power supply, and the electro-optical crystal is suitable for rotating the phase of the input polarized light for pi/2 when voltage is applied to the electro-optical crystal; the reflective structure makes the polarized light output by the electro-optical crystal return along the same way; the first polarization device is suitable for selectively outputting the polarized light received from the electro-optical switch according to the polarization state of the polarized light; and the electro-optical driving power supply periodically applies 1/4 wave voltage to the electro-optical crystal, so that the polarization state of the polarized light output by the electro-optical crystal is periodically changed. By the device, the optical regulation of output power/energy can be realized, and pumping source current is not required to be changed, so that the influence of a thermal focal length change in a laser on system stability is avoided.