On the basis of data from the geomagnetic network and ionospheric observatories in China, the geomagnetic low point displacement, diurnal variation anomalies of geomagnetic field and ionospheric f0F2 （critical frequency of F2 layer） anomaly phenomena before large earthquakes have been summarized. Anomaly distribution and characteristics of geomagnetic variation field and ionospheric f0F2 before Mani MS 7.5 earthquake on Nov.8, 1997, and Kunlunshan MS 8.1 earthquake on Nov. 14, 2001, have been comparatively studied. The result shows that the temporal spatial characteristics of short term and imminent anomalies of geomagnetic field and ionosphere are consistent. The obvious anomalies of diurnal variation of geomagnetic field near epicentral area and ionospheric f0F2 at Lhasa observation are found. The geomagnetic low point displacements appeared about one month before the two great earthquakes, the boundary lines of sudden change passed through the epicentral area.