On the premise of studying the physics basis produced by unstability of source system,and according to the modulation model,we put forward the suggestion to make short term predic-tion using correlation between external agencies and frequency fluctuation intensification of small modulation earthquakes,we also suggest two indexes λ₁,λ₂ to discriminate the unstability of source system,the former is the unique factor modulation index,and the latter is multi-factor modulation index, or multi-factor modulation intensification index.The site prediction of strong earthquake should be decided according to the conjugate belt of small earthquake activi-ty in the stereoscopic model which appears immediately before earthquake and goes cross the future epicenter.