A fiber optic fabric sensor for respiratory monitoring of smart clothing is investigated. The sensor consists of a multilayer composite of a stretch-sensitive optical fiber, thermoplastic polyurethane elastomer (TPU), and an elastic fabric. A heating-based sizing method for the multibending cascade structure of polymer optical fibers is proposed to prepare stretch-sensitive optical fibers with precise characteristic dimensions. Using the TPU material, the bonding of stretch-sensitive optical fibers, TPU material, and fabric is realized by ironing to form a laminated fabric sensor. The prepared fabric sensor has no bubbles and wrinkles between the layers, has good production repeatability, and can be connected with garments without sewing to enhance their comfort and aesthetics. Experiments show that the sensor has a strain coefficient of up to 71.01, a stretch rate of up to 83%, a hysteresis error of <12%, and a unidirectional stretch-sensing capability. The designed respiration monitoring sample garment is put through its paces, and the measurements revealed that it is capable of capturing a clear respiration waveform under a variety of breathing frequencies, wearer postures, and movement states. The maximum error of respiration rate is <2 times/min and the average error is within 0.8 times/min.