Abstract: Strong earthquakes with magnitude M7 do not occur in quiet places,but in areas with a certain seismic background. Study of the rules of the background seismicity may provide information for judging the potential areas of seismic sources of strong earthquakes. This paper deals with eighteen earthquakes with magnitude M7 in North China,studies the seismic activities before and after each of these large shocks,and proposes four rules for background seismicity of strong earthquakes in North China:1. Before a strong earthquake there are always background earthquakes whose magnitude is larger than six (M6). 2.There are informative (signal) earthquakes which occur before the main shock.3.A phenomenon of seismic postponing exists between two strong earthquakes whose epicenters and occurrence times are close to each other.4.A strong earthquake can reduce the seismic activity of its vicinity. Based on these four rules this paper judges the potential areas of seismic sources of strong earthquakes for the period A. D. 1500-1730 in North China.