The northern region of Qinghai-Xizang plateau is divided into four separated seismicity areas according to the regional geological structures,stress field and dynamic environment,they are Qilian mountain seismic belt,southeastern Gansu province,Caidam-Gonghe block and Kuma seismic belt.The starting times for the minimum earthquake magnitude with complete recording catalogue in every areas are confirmed on the base of b-value and seismic frequency.The alternative characteristics of active and calm periods for moderate-strong earthquakes in each area are concluded.Finally,the quantitatively characteristic parameters of earthquakes activity which have magnitudes bigger than moderate-strong event in average,active and calm periods are calculated separately,and also the state parameters of small event activity in three periods are obtained for the four areas.