Critical rainfall thresholds can be the key to ensuring effective debris-flow forecasting. They are significant for the study of the trigger mechanisms of debris flows, for the forecast of the characteristics of future events, and for the development of engineering guidance for mitigation. Using a hydrological approach, we first calculated the flood peak discharge at different frequencies and then the corresponding rainfall thresholds for the initiation of different scales of debris flows in Zhouqu County, China. This was followed by the establishment of a functional relation between the intensity and duration of rainfall events that trigger debris flows of different warning levels in two initial soil conditions (dry and moist). This, in turn, yielded four warning levels and two preliminary warning levels. For the two early soil conditions (dry and moist) in the Sanyanyu Gully of Zhouqu County, the Level I (red) warning values for rainfall triggering debris flow are 56 and 51 mm h−1, respectively; the Level II (orange) warning values are 41 and 38 mm h−1, respectively; the Level III (yellow) warning values are 32 and 30 mm h−1, respectively; and the Level IV (blue) warning values are 24 and 22 mm h−1, respectively. The Level V preliminary warning values are 17 and 16 mm h−1, respectively; and the Level VI preliminary warning values are 10 and 9.5 mm h−1, respectively. The rainfall intensity and duration were found to exhibit a power function relation, I = αDβ, where the values of α and β vary with the warning levels. Rainfall events capable of triggering debris flows in the new thresholds and intensity–duration relations presented here can be used for forecasting purposes and in operational geohazard warning systems. These research results also provide a scientific basis for regional hazard mitigation and reduction in Zhouqu County.