An optical fiber weak magnetic sensor with sensitivity of 10-5 Oe. is reported. The device uses metal-glass as magnetostrictive material and an Alternating Current Phase Tracking (PTAC) homodyne system for phase detecting in optical fiber M-Z interferometer. Nulling technique eliminates any problems arising from the magnetic hysteresis effects of the magnetostrictive material which affect its magnetometer output. The theory and design method of the transducer and the phase detecting system are analyzed in detail.