It is found that the phenomenon of concentration of medium-strong earthquakes before M ≥ (7.0) exists in 20th century in Yunan province.A flexible time prediction of main earthquakes can be made on the basis of the features of concentration of medium-strong earthquakes, and the epicenter of potential main earthquake can also be predicted through the study combining with "the earthquake in quiescence".At the end, the mechanism of relation between the foreshocks and main shock is discussed.