Abstract: The geomagnetic transform function represents the lateral inhomogeneity which could be used to study the changes of the underground electricity structure during the seismogenic process. In this study, the geomagnetic transform function and variation characteristics of geomagnetic induction vector for each measuring point of the geomagnetic array in northwestern Yunnan Province before and after the Eryuan M_S5.5 earthquake on March 3, 2013 are analyzed by using the bounded influence estimation geomagnetic transform function. The results show that the absolute values of the real part of the transform function for Yunlong station and Eryuan station undergo a process of increase, turn and recover in more than one period during 1−2 months before the earthquake. The amplitude of the abnormal change is more than one time of the standard deviation. Before the earthquake, there is no obvious abnormal change in the transform function of the other measuring points which are far away from the epicenter or theaftershock area. The results show that the abnormal range of the geomagnetic transfer function before the Eryuan M_S5.5 earthquake is less than 50 km, which may be related to the spatial distribution of the aftershock area.