We propose a pH sensor based on calcium alginate (CaAlg) hydrogel-coated micro-nano fibers (MNFs) for the high sensitivity detection of sweat pH. The basic principle is that the change in refractive index induced by the swelling and contraction of the CaAlg hydrogel caused by a change in the solution pH promotes a change in the effective refractive-index difference of the MNF bimode interference, thereby causing a drift in its interference wavelength. The effect of the CaAlg hydrogel concentration on the pH sensitivity of the sensor was analyzed, and the performance indices related to sweat-pH sensing were evaluated. Experimental results show that the sensor can detect the pH of the aqueous solution in the range of 2 to 9 as well as achieve an ultrahigh sensitivity of -4.87 nm/pH. The average sensitivity for artificial sweat in the pH range of 4.4?8.8 is approximately -5.83 nm/pH, the linear coefficient exceeds 0.984, the response time is less than 2 min, and the stability is 95.000%?99.286% within 100 min. The proposed MNF-based pH sensor offers high sensitivity, a simple structure, stable performance, and easy fabrication, as well as demonstrates high feasibility and application potential for the detection of sweat pH.