With the advantages of high sensitivity,simple structure,and low cost,the multi-longitudinal mode beat frequency technique has attracted considerable attention since it was first proposed in 2010. The fundamental mechanism behind this technique is that numerous multi-longitudinal modes within the fiber laser bandwidth are stimulated and beat with each other to generate a Beat Frequency Signal (BFS). The Free Spectrum Range (FSR) of the BFS is related to the propagation time in the fiber laser cavity,thus the applied parameters on fibers can be recovered by tracking the change of selected BFS. Compared with traditional optical fiber sensors interrogated in the optical domain,the BFS can be detected in the frequency domain. Benefitting from the large frequency gap between the optical signal and frequency signal and the mature electronic instrument,the interrogation resolution and speed can be enhanced greatly. The sensitivity of BFS is proportional to the selected frequency. In other words,in order to improve the sensitivity,high-frequency BFS should be chosen,which brings instability and low measurement range.The vernier effect is an effect method to improve the sensitivity in optical fiber interferometers. The vernier envelope can be generated by combining two interference spectrums with different FSRs. Hence,the measurements can be recovered by monitoring the vernier envelope with an enhanced sensitivity. Up to now,researchers have applied the vernier effect to measure parameters,including temperature,strain,refractive index,etc. It should be noted that the vernier effect magnifies the sensitivity but also the measurement error,resulting that the vernier effect is appropriate for the applications with a damage threshold. Essentially,the vernier effect is formed by two periodic signals with different periods,it also can be generated in the frequency domain.In this paper,the vernier effect is introduced to the multi-longitudinal mode beat frequency technique to enhance the sensing sensitivity. To verify the proposed method,optic-fiber axial strain measurement is experimentally carried out. Two linear-cavity fiber lasers with different lengths are formed by a pump laser (pump),a Fiber Bragg Grating (FBG),a section of Erbium-Doped Fiber (EDF),and two Faraday reflector mirrors. With the help of the pump and the EDF,FBG-based laser with the same wavelength of the FBG is emitted and picked by a Photodetector (PD) to achieve photoelectric conversion. Benefitting from the large bandwidth of the FBG-based laser,numerous longitudinal modes are emitted and beat with each other to generate BFS,which are recorded by an Electrical Analyzer (ESA).Firstly,the axial strain sensing characteristic of cavity 2 is investigated. Breaking FRM 1 and FRM 2,the BFS with FSRs of 6.97 MHz and 7.52 MHz with signal-to-noise ratios of 20 dB are generated. Keeping FRM 2 connected and FRM 1 disconnected,a section of single mode fiber with a length of ~55 cm in cavity 2 is fixed between two move stages. The strain is applied to the fiber from 0 ~1 090.8 με with a step of 90.9 με by pulling one move stage. The BFS with the frequency of 1.506 94 GHz generated in cavity 2 is chosen as the tracking signal. Results show that with the increased axial strain,the selected BFS shifts to a lower frequency range and the slope can be calculated as -1.228 kHz/με by linear fitting test data,and the R² value is 0.998.Secondly,the characteristic of the vernier is investigated. By connecting FRM 1 and FRM 2 at the same time,an obvious vernier effect is generated. In order to get the upper envelop curve of the BFS,extremum points are extracted and polynomial fitting is performed. The bandwidth is enhanced and the FSR is 95.06 MHz,agreeing well with theoretical analysis. Thirdly,the axial strain sensing sensitivity is experimentally tested. The strain is performed as above and the envelope with frequency of 1.524 24 GHz is selected as the tracking signal. Results indicate that the envelope shifts linearly with the increased axial strain and the sensitivity is -17.23 kHz/με with an R-square of 0.993. Compared with the sensitivity in cavity 2,the amplification is 14.09,agreeing well with the calculated value. Meanwhile,Root Mean Square (RMS) method is employed to get the frequency of the envelope,and the results shown that the sensitivity is -18.47 kHz/με,which is slightly larger with the peak sampling method.Finally,the stability in two situations is tested. In this step,the axial strain applied on fiber is kept at 90.9 με and the frequency change is recorded every 1 minute in 10 minutes. Experimental results show that the maximum frequency fluctuations are ±3.13 kHz and ±101.17 kHz,corresponding to measurement errors of ±2.55 με and ±5.74 με,respectively. It proves that the vernier not only enhances the sensitivity but also enlarges the measurement error. This study provides an important reference to enhance the sensitivity of the multi-longitudinal mode beat frequency technique and has a potential application in other parameters sensing scenes.