Forecasting and explaining the sudden changes of tropical cyclones is one of the most difficult problems in the present meteorology. This research has been conducted for thirty years and found that it is all trigger by the depressed tide generating forces resonance (TGFR) that the abrupt changes of the tropical cyclones' strength and the torrential rainstorms after their landing. That causes all the abrupt changes of tropical cyclones' way and moving speed: the TGFR bring on the sudden changes of subtropical high outside the tropical cyclones,this arouses the leading airflow unexpectedly change its direction or strength.