A fiber laser refractometer based on an open microcavity Mach-Zehnder interferometer (OMZI) is proposed. The open microcavity is constructed by embedding a segment single-mode fiber (SMF) into two multi-mode fiber (MMF) joints with lateral offset for liquid sample, which has the advantages of micro sensing element and easy fabrication. The transmission characteristics of the MMF-assisted OMZI are investigated by simulating and manufacturing the OMZIs with different microcavity lengths and offset distances. By inserting the MMF-assisted OMZI into the erbium-doped fiber ring laser (FRL) cavity, the lasing wavelength can be used to detect the refractive index (RI) change of the medium in the microcavity. Experimental results show a high sensitivity of -2953.444 nm/RIU within the measurement range of 1.33302∼1.33402. More importantly, with the combination of OMZI and FRL, the proposed fiber laser refractometer realizes ultra-low detection limit (DL) and high-quality factor Q, which are two orders of magnitude better than that of previous reports.