An optical fiber Sagnac loop pH sensor based on the swelling stress of hydrogel is developed, selecting polyacrylamide hydrogel with favorable swelling performance as the sensing medium, considering the differential swelling of hydrogel in solutions with varying pH values. The differential swelling of the hydrogel in various pH solutions induces a change in the axial stress of the polarization-maintaining fiber to lead to spectral shifts in the Sagnac loop output. By characterizing the spectral shift, pH sensing is achieved. The sensor exhibits a linear response within the pH range of 2 to 12, with a sensitivity of 0.947 nm/pH. Experimental results show that it has good robustness, stability and repeatability.