In this paper, the total geomagnetic intensity data and the difference of total geomagnetic intensity between two stations were used to eliminate short-term change of geomagnetic field, and at the same time the influence of long-term change was weakened, so that the annual change is a linear change. Based on this, the regional correction value of the residual long-term Change was obtained by the method of trend surface. After processing the data, it was shown that before the great Tangshan earthquake there were. probably anomalies for more than one and a half years, with change amplitude about 10 gels. Within the network of stations, the other earthquakes with magnitude of about 6 were also accompanied by anomalies of total geomagnetic intensity.