On the basis of the giant magnetoimpedance (GMI) effect found in the amorphous wire, and with the utilization of the CoFeBSi amorphous wire as the sensing material, a novel differential-type high sensitively sensor adopting the technology of orthogonal lock-in amplifier was designed. Firstly, the concept of the GMI effect and the magnetic sensing head based on the double-sensing-elements and differential-type structure, were briefly instructed. Secondly, the principle of the conditioning circuit of the sensor based on orthogonal lock-in amplifier was analyzed in detail. Thirdly, based on the impedance magnitude and phase characteristics of the amorphous wire, an approximate algorithm was put forward to calculate the envelope of the output of the orthogonal lock-in amplifier for simplifying the arithmetic circuit. Experiment results show that the sensitivity of the sensor can achieve about 748mV/Oe with the full measurement range of -2.0Oe ～ +2.0Oe, and the linearity error is about 0.98%FS, while the average noise power spectral density is about 0.8nT/Hz^1/2.