This Research Reports an Ultra-Sensitive Miniaturized Optical Fiber Strain Sensor based on a One-Step Assembly-Free Structure. a 10 µm Thin Asymmetrical Taper Waist of a Single Mode Fiber (SMF) Was Designed and Fabricated to Form a Michelson Interferometer (MI). the Sensor Has Been Experimentally Demonstrated for Strain Measurements with Long-Term Stability. the Sensor Provides a ∼14 DB Fringe Visibility with a High Extinction Ratio in the Reflection Spectrum. Experimental Results Indicate that the Sensor Exhibits Maximum Strain Sensitivity of −39.77 Pm/µε in the Measurement Range of 0 to 1600 µε. the Sensor Shows Promising Results in Terms of Stability and Repeatability. the Proposed Sensor Requires Only One-Step Fabrication with Great Reproducibility And, Therefore, Can Be Easily Mass-Produced with a Less Stringent Requirement in Fabrication Equipment. a Highly Sensitive Response for Measuring Axial Strain Makes the Sensor a Viable Contender for a Number of Specialized Niche Applications.