Ground vibrations provide relevant flow conditions and may be used to warn of potential debris flow. To construct warning methods for debris flow, geophone ground vibration recordings from four debris flows at Ai-Yu- Zi creek were analyzed. Based on time-frequency analysis, the effect of time span and frequency band were examined, and the representative indexes of accumulated power and accumulated power slope were extracted. With extracted indexes, we used ambient noise and scale factors to define warning thresholds. The warning times for debris flows were calculated with floating warning thresholds and then successfully validated by two events with a maximum error of 4.2 s.