The seismicity factor A-value is defined by synthesizing the seismicity precursors in time, space and magnitude. The seismicity data of moderate or small earthquakes during 1972~1996 in northern China are used in spacial scanning of seismicity factor A-value, studying mid-short term change of the A-value before the earthquakes. The result shows that in mid-short term, the midterm anomaly zones of A-value that appeared about 2~3 years before a lot of strong earthquakes may contract or disappear obviously, and the zones may obviously extend again about 1~3 months before some earthquakes. Some problems about mid-short term earthquake prediction by using the A-value are also disscused.