First,　the　actuality　of　fiber　sensors　in　homeland　and　abroad　are　outlined　in　this　paper.　And　the　characteristics　and　transfer　matrix　of　high　birefringent　fiber　(HBF)　are　introduced.　After　deduction　of　transfer　matrix　of　the　HBF　Sagnac　loop　mirror,　a　conclusion　that　the　frequency　of　optical　transmissibility　will　not　change　with　the　wavelength　change　basically　is　obtained,　and　it　is　proportional　to　the　length　of　the　HBF　in　the　loop　mirror.　After　the　theoretical　conclusion　is　verified　by　spectrometer,　the　HBF　Sagnac　loop　mirror　application　to　sensor　and　demodulation　are　discussed.　Because　the　frequency　of　optical　transmissibility　is　not　change　following　the　wavelength　change,　and　it　is　proportional　to　the　length　of　the　HBF　in　the　loop　mirror,　we　adopt　the　counting　method　for　wavelength　demodulation.　Using　a　long　HBF　Sagnac　loop　mirror,　through　the　recording　number　of　changed　periods　of　the　optical　transmissibility,　the　wavelength　variation　can　be　judged.　This　method　gets　high　resolution,　it　even　can　be　better　than　0.01nm　which　is　the　higher　precision　of　spectrometers.　The　HBF　is　also　more　sensitive　to　temperature.　If　changing　the　temperature　of　the　HBF　in　the　loop　mirror　and　keeping　the　wavelength　of　input　light　invariable,　the　intensity　of　output　light　also　changes　in　period.　For　a　short　HBF　Sagnac　loop　mirror,　the　period　gets　longer,　and　the　range　of　temperature　change　is　included　in　its　half　period.　Using　the　short　HBF　Sagnac　loop　mirror,　we　have　achieved　the　temperature　measurement.　The　material　and　devices　including　HBF,　Pin,　circuit　of　amplification　and　Mega8　A/D　converter　are　only　used　in　the　two　methods　as　mentioned　above.　The　structure　is　simple　and　the　cost　is　low.