The invention provides an all-fiber F-P hydrogen sensor and a preparation method of a hydrogen sensitive film of the all-fiber F-P hydrogen sensor. The sensor is structurally composed of an all-fiber F-P resonant cavity, a sensitive film orientated expansion limiting piece, a fixing part, a sensor packaging part and the like; a Pd-Ag alloy sensitive film with certain thickness can be directly deposited on the outer layer of an all-fiber F-P resonant cavity sensitive glass sheet at low temperature by adopting a pulse laser depositing process; one surface of the sensitive film orientated expansion limiting piece is only in contact with the sensitive film through the fixing part and a limiting expansion block is made of a porous ceramic material, so that hydrogen can be absorbed; when the sensitive film absorbs hydrogen to have orientated expansion, the cavity length of F-P is compressed and reduced; and the cavity length of the all-fiber F-P resonant cavity is demodulated so that the change of the hydrogen concentration is measured. By adopting the preparation method, the low-temperature film plating of fibers is guaranteed; and the sensor is simple in structure and convenient to prepare, and industrialized production is easy to realize.