In the development of GMI magnetic sensor, weak magnetic signals are always submerged in the circuit intrinsic noise. Since traditional peak detecting method is impossible to detect GMI sensor’s output when its SNR is lower than 0dB，a new method is presented to detect the GMI probe’s weak signals（SNR<0dB）. The LMS algorithm was applied firstly to extract the characteristic parameters of the weak signals, and then the cross-correlation function between the characteristic parameters and the idea ones was calculated. Finally the maximal correlation values were regarded as weak signals to be detected by means of calibration. The simulating results indicate that this method can not only detect but also measure the magnetic field by the corresponding relationship of magnetic field intensity and the maximal correlation values, even the SNR of GMI magnetic sensor’s output is 10dB.