A novel method for active fiber refractive index measurement based on beat frequency interrogation of the multi-longitudinal-mode distributed Bragg reflector (DBR) fiber laser was presented. Based on beat frequency interrogation, the measurement of erbium-doped fiber refractive index had been taken by monitoring the beat note between adjacent longitudinal modes. This method was simple of structure, and low cost compared with the conventional wavelength interrogation approach, etc. The measurement method had been analyzed theoretically and tested experimentally. By utilizing a uniform cantilever beam (UCB) for axial strain on the oscillation cavity, strain dependence of fiber refractive index had been measured. Experimental results indicated that erbium-doped fiber refractive index linearly decreased with strain increased, and its coefficient of determination reached to 0.9971. The strain coefficient of fiber refractive index was about -6.3086×10-7 (με)-1, basically in agreement with earlier related studies.