According to limit time method,the h' values of 11 groups of M_S6.5 strong earthquakes since 1970 in Yunnan province are calculated. The results show:h' anomalies appeared before 11 groups of M_S6.5 strong earthquakes with sudden decrease of 28%(the average amplitude), meaning quiet anomaly of seismicity. It is the key index for make prediction. In 11 groups of strong event, 8 of the seismic process show the seismogenic mode of activation-quiet-main earthquake, which posses 73% of all; 3 are activation-quiet-activation(short-term)-main shock mode, which posses 23%; It was displayed clearly for quiet or activation of seismicity before strong earthquakes. Making forecasting to 11 groups of strong earthquakes suggests that forecast time is about 5 months.