A polyaniline film micro-nano optical fiber liquid hydrogen ion concentration probe is proposed， and the hydrogen ion concentration sensing characteristics of this fiber probe are investigated. The sensing part consists of a single-mode fiber， a small-core-diameter fiber and a single-mode fiber pair fusion spliced together， and polyaniline material is coated on the small-core-diameter fiber as a sensitive film. Firstly， a theoretical analysis of the sensing principle of the fiber optic probe and the structure change mechanism of the polyaniline main chain was conducted. Then， the effect of different film thicknesses on the sensing characteristics of the fiber optic probe was verified and analyzed by hydrogen ion concentration sensing experiments. Finally， experimental interference tests on the change of liquid refractive index were completed and the response-recovery time and stability performance of the probe were evaluated. The experimental results show that the main chain structure of polyaniline changes under the action of hydrogen ions. The interference spectrum of the fiber optic probe drifts in the short-wave direction with the increase of hydrogen ion concentration， and the detection range increases with the rise of polyaniline film thickness， while the sensitivity and linearity decrease significantly. The sensitivity of the fiber optic probe is -15.74 nm/mol/L for the hydrogen ion concentration range of 10^-6~10^-1 mol/L， and the response and recovery time are 25 s and 35 s， respectively. The experiments verify the change of polyaniline film’s optical properties after reacting with hydrogen ions and provide a new method for liquid hydrogen ion concentration detection. This fiber optic hydrogen ion concentration probe also has the advantages of high concrete film strength， simple fabrication， and low cost.