A fiber inline interferometric refractive index (RI) sensor consisting of a microchannel and a fiber taper is proposed in this letter. The microchannel is fabricated by combining femtosecond laser micromachining and arc fusion splicing. No subsequent chemical etching process is needed. Three sensors with microchannel widths of 4, 8, and 10 \mu m are prepared. The sensitivity in the RI range from 1.33 to 1.35 is up to ~361.29 nm/RIU at the microchannel width of 8 μm. The sensitivity is ~20 times greater than that of the paired taper-based MZI sensors and long period fiber grating pair MZI sensors.