The invention discloses a high-speed railway subgrade void monitoring system based on a fiber grating technology. Demodulation equipment is installed on the surface layer of the high-speed railway subgrade, a supporting layer is arranged at the upper end of the subgrade surface layer, a mortar cushion layer is arranged at the upper end of the supporting layer, a track plate is arranged at the topof the mortar cushion layer, a steel rail fastener is installed at the upper end of the track plate, and void equipment is installed at the upper end of the supporting layer and conducts signal transmission with the demodulation equipment. According to the invention, when a sensor probe moves downwards due to the void of the measured high-speed railway subgrade segment, the subgrade void amount iscalculated by measuring the wavelength offset of the fiber grating in the sensor, so that the continuous monitoring of the high-speed railway subgrade void is realized.