Abstract: In this paper, by using the method of trend surface analysis, the data of mobile geomagnetic measurements in the joint area of provinces of shandong, Jiangsu, Anhui and He-nan from August 1976 to November 1979 are analysed. Preliminary analysis shows that the geomagnetic variation in the are a is of normal magnetic variation. The trend part of the variation is the secular geomagnetic variation. The secular variation rates are not entirely the same for each locality in the area. But the spatial distribution of the rates reveals certain regularity. The character of the secular variation agrees with that obtained on the basis of other geomagnetic data. No anomalous local secular geomagnetic variation has been found in the area. The accuracy of determination of this observational system is about 3.0 r . The variations of the residuals of most of the points are within the observational error .But before and after the Guzhen (固镇) Earthquake (Ms=5.0) on March 2, 1979, the variations of residuals of the points around the epicentre-seem to be anomalous, exceeding the observational error.