A fiber sensing system based on laser intracavity modulation with a low detection limit for refractive index sensing is proposed. A kind of all-fiber multimode interference structure based on single-mode-no-core-single-mode fiber is inserted in a fiber laser ring cavity as a loss-modulation device. The fiber laser intracavity modulation technology is used to obtain high-sensitivity sensing signal with high signal-to-noise ratio and narrow full width at half maximum. Depend on these characteristics, the low detection limit of the sensing system can reach up to 7.3×10 -7 RIU. The sensing system with output stability and low temperature cross-sensitivity has a great potential in high-accuracy biochemical sensing and marine environmental monitoring.