In this paper, the authors present a quantitative prediction index called seismic force scale σ values. The authors introduced the method of calculating the σ value in detail, and then studied the variation of σ values in time and space before strong earthquakes. The results show: 1) the relationship between σ values and magnitude ML follows lgσ=0.98ML+6.93,and σ is a physical quantity that characterizes the average driving force of fault in earthquake source; 2) in the Yunnan area, σ values appeared cyclic similar to the sine wave; and 3) high-value anomaly in σ values appeared 2～4 months before the main shock in the larger region.