An innovative label free optical fiber biosensor based on Mach-Zehender interferometer for Bovine Serum Albumin density detection will be introduced. The fiber biosensor employs a micro-cavity within the single mode fiber to form Mach-Zehender interference. The device is fabricated by means of femtosecond laser micromachining and chemical etching. Such a fiber interferometer exhibits an ultrahigh refractive index sensitivity of -10055 nm/RIU. The biosensor exerted the BSA solution concentration sensitivity of -38.9 nm/(mg/mL) and detection limit of 257 ng/mL respectively.