A kind of optical fiber laser displacement sensor with multi longitudinal mode beat frequency was proposed and designed. The sensor could be realized by using the beat frequency signal between different modes in the resonator to monitor the small displacement of the object. The sensor used a strain sensing substrate structure to effectively protect the optical fiber and buffer the deformation caused by stretching and compression, and increase the sensitivity of the sensor. At the same time, the original multi longitudinal mode beat fiber laser was improved, which made the system get higher signal-to-noise ratio. The measuring principle of multi longitudinal mode beat sensor was described, and strain gage was designed and built. In the experiment, four different beat frequency signals were selected as sensing signals. The experimental data show that the frequency shift is basically linear in the range of 0 to 30 mm, and the highest linear fitting degree can reach 0.999 4, which is in agreement with the theoretical results. Because the precision of the stretching platform is limited, the measuring accuracy of the sensor is 1×10-3 mm. Repeated experiments show that the sensor has good stability.