Laser self-mixing technique has the advantages of high sensitivity, easy collimation and non-contact measurement. In a multi-longitudinal mode laser self-mixing interference system, the self-mixing signal waveform changes periodically with the external cavity length. According to this physical phenomenon, a novel scheme for measuring the free spectral range (FSR) of laser using a three-longitudinal mode laser self-mixing vibration sensing system is proposed. Combined with the interference mixing theory and three-mirror cavity theory, the theoretical model of measuring laser FSR based on a three longitudinal mode laser self-mixing vibration sensing system is established for the first time, and the corresponding numerical simulation is carried out. Results show that the FSR of the pigtailed laser diode is affected by the external environment, and its variation range is from 163.93 GHz to 175.64 GHz. The displacement resolution and frequency resolution of the system is 0.01 mm and 1.91 GHz respectively. Due to the excellent merits of high resolution, compactness and low-cost, it is expected that the system can be applied to FSR measurement of various types of lasers.