Abstract: We mapped out eighteen historical great seismic gaps based on characteristic of spatial distribution of generalized pre-earthquake of China great earthquake.Studying content includes three section:The section 1 is to discuss emphatically the characteristics of great seismic gaps in continental region of China, i. e. universality of the earthquake-brewing and change of earthquake activity in gap. The relationship between seimic gap and tectonics has been discussed summarily.The section 2 is to determine roughly quantitative sign for magnitude and time of occurring major earthguake to come in the light of empirical relations achieved by statistics, for example:1. The relation between area and long axis of gap and magnitude2. The relation between the amount of total earthquake energy in gap and magnitude3. The relation between the whole area of fault plane and magnitude.The section 3 is to try to do earthquake forecast of moterate-long term. Finally we suggest the six gaps of the potential mejor earthquake be put to the test in predicting the earthquake.