In order to detect gadolinium nitrate with high sensitivity, a fiber ring cavity laser sensor based on micro-nano fiber interferometer is studied in this paper. A kind of micro-nano fiber interferometer with double cone structure is prepared. The strong evanescent field of micro-nano fiber can interact effectively with the external environment or surrounding medium, and the evanescent field increases with the decreasing of the diameter of fiber biconical structure. The sensitivity of refractive index sensing is also significantly improved. The experimental results show that when the diameter of micro-nano fiber is 20 μm, the refractive index sensitivity of the sensor is 1925 nm/RIU; for the mass concentration of gadolinium nitrate solution, the sensitivity of the sensor is as high as 0.172 nm/(mg·mL^-1). In addition, the fiber ring laser sensor has the advantages of high sensitivity and compact structure, and has a broad application prospect in biological modification and chemical sensing.