Referring to the M≥5.0 earthquakes from 1500 to 2000 and the M≥4.0 earthquakes from 1970 to 2009 in North China, the authors analyze the Benioff strain using wavelet transform in various scales. Analysis shows that on a century-scale the M≥5.0 earthquake-activity period has just finished and now the stress is accumulating. And since 1970 there has existed activity cycle of about 7 years for the M≥4.0 earthquakes. Now the M≥4.0 significant earthquake-activity period is beginning in North China.