Optical heterodyne interference systems have high signal frequency. The tranditional oversampling demodulation method has the disadvantages of high cost and large amount of data, so it is inconvenient for real-time demodulation monitoring. In this paper, an undersampling digital orthogonal demodulation method is studied to reduce the sampling frequency of a heterodyne system, so as to meet the demands of fast demodulation and real-time monitoring in vibration sensing. Firstly, the undersampling principle of a heterodyne interference system is analyzed theoretically to obtain the condition that the sampling frequency must meet, that is, the minimum sampling frequency should be more than twice of the signal bandwidth. Then undersampling demodulation is verified experimentally. For a vibration with a 0.5 V piezoe-lectric transducer (PZT) excitation voltage and an 80 MHz heterodymium frequency, the phase could be demodulated with sampling frequency as low as 20 kHz‒10 MHz. The largest demodulation error is 0.3%, and it does not increase with the decrease of sampling frequency. The influence of sampling frequency on measurable PZT excitation voltage range is studied. Higher sampling frequency leads to larger voltage range, and the linear demodulation intervals corresponding to 50, 100, and 200 kHz sampling frequencies are 1‒7, 1‒14, and 1‒20 V, respectively. Undersampling digital orthogonal demodulation is believed to have extensive application prospects in optical heterodyne interference systems due to its low-cost and fast demodulation.