A novel capillary-fiber-based all-in-fiber platform is proposed and experimentally demonstrated for microfluid sensing application. The miniature optofluidic channel consists of a capillary fiber with a length of about sandwiched between two coreless fibers. The proposed microfluidic structure is precisely fabricated by means of visible fiber-micromachining technique based on ultrasonic cutting. This microfluidic platform exhibits superior-performance for low-range refractive index sensing and a sensitivity of 13582.7163 nm/RIU with a low detection limit of ^-5 RIU is achieved in the low-range fluidic liquid refractive index of 1.3332-1.3397. Moreover, the all-in-fiber platform possesses the distinct advantages of strong light-matter interaction, good stability, reusability, simpler fabrication, and easy integration, which makes it a promising application for tackling a variety of low-trace biochemical analysis problems, like cell biology, virus and bacterial, nucleic acid and protein detection.