The invention discloses a hydrogen concentration detecting device which comprises a hydrogen sensing head (10), an integrated optical signal module (2) and a signal processing unit (3) which are connected mutually, wherein the hydrogen sensing head (10) comprises a polarization-maintaining photon crystal sensing optical fiber (1) plated with a palladium membrane, a first optical fiber coupler 4a, a protecting sleeve (5), an optical fiber fixing frame (6), an optical adapter (7) and a tail fiber jacket (14); and the integrated optical signal module (2) comprises a light source (8), a spectrum receiving device (9) and a second optical fiber coupler 4b. An optical path adopted by the device is reliable and stable in performance; the adopted polarization-maintaining photon crystal optical fiber plated with a palladium membrane is good in material consistency and has good temperature insensitivity; and the membrane plating length of a hydrogen-sensitive palladium membrane can be designed according to the requirements on sensitivity so as to realize the high sensitivity detection. The device is not sensitive to the change of temperature, and is simple in system structure, convenient to operate, strong in interference resistance and high in precision.