A polarization-maintaining photonic crystal fiber (PM-PCF) hydrogen sensor based on Sagnac interferometer is proposed. A sensing model is established and experimentally studied and tested. With the facing target sputtering process, Pd/Ag composite film is deposited on PM-PCF fast and uniformly. The birefringence of PM-PCF can be modified by the deformation of film after absorbing hydrogen, realizing the measurement of hydrogen concentration by detecting the wavelength shift of one specific valley. The results show that good repeatability is obtained within the measuring range of 4%. This sensor′s sensitivity is higher at low hydrogen concentration and the wavelength shift is 1.307 nm when the hydrogen concentration increases from 0% to 1%. The birefringence of the PM-PCF is not sensitive to temperature, restraining the interference brought up by the fluctuation of environment temperature.