With the aim to fit the sin wave curve of a strap-down north seeking system and to calculate the angle between the start position and the real north accurately,the principles of the system are introduced,and the factors influencing the measuring precision are analyzed.Then,the least square method and the Back-Propagation(BP) neural network are adopted to fit curves separately.Experimental results indicate that the BP method is preciser than the least square method,for its error sum is 0.23 μV and the squared error of residual error is 1.3 mV.Moreover,for phase angle measurement,both methods can get the same average values in six measurements,but the squared error of BP neural network is 8″,which is more accurate than that of the least square method.These results show that the proposed BP method can satisfy the precision requirements of curve-fitting north seeking systems.