Debris-flows not only cause a great loss of property, but also kill and injure people every rainy season in the mountainous regions of China. In order to reduce hazard and risk, several methods of assessing rainfall thresholds have been provided at present, based on statistical models. However, the limited rainfall data with debris-flow occurrence or non-occurrence makes threshold analyses very difficult. This paper, therefore, presented a kind of comprehensive threshold consisting of pore-water pressure from Terzaghi theory, and rainfall factors from frequent usage for predicting debris-flow occurrence. Rainfall and pore pressure data has been collected in a number of locations in Wenjiagou gully to assess critical rainfall and pore pressure values for debris flow initiation. The three-level early warning criteria (Zero, Attention, and Warning) has been adopted and the corresponding judgement conditions has been defined based on monitoring data in a real-time way. Finally, it is suggested that the combination of these two critical values might be a useful approach in a warning system for safeguarding of population in debris-flow prone areas.